It was a superb performance from the Italian, who wasted a late chance to make it four.
Pazzini certainly stunned the 60,000 crowd with his early strike and it took England a while to settle into the game.
Bentley had restored parity by the break - but it was in the second half that the game really came to life.
Giorgio Chiellini failed to intercept Derbyshire's cross and Routledge scored with a crisp strike before Pazzini turned Antonio Nocerino's strike past Lee Camp.
England led again through Derbyshire but Pazzini levelled after picking the ball up on the right and shooting low across goal.
THE ACTION AS IT HAPPENED (all times GMT)
By Paul Fletcher
"A great first game for the new stadium, it had everything and the atmosphere was breathtaking. Sad that England did not win." Toby, Oxford, via text on 81111
"It was disappointing to concede like we did but we have learnt a bit. There are areas we need to work on but as an opening game it was a good exercise." England Under-21 stand-in coach Nigel Pearson
1420: Full-time England U21s 3-3 Italy U21s
The final whistle sounds - if every game is like this then Wembley really will become a legendary venue.
1418: Almost at the final whistle. Certainly been a thrilling opener.
"The pitch has cut up all over the place there is a lot of work to be done on it but the main thing is that Wembley is up and running." Chris Waddle, BBC Sport
"My wife went into labour 20 minutes before kick-off so I have had to miss the game. If it is a boy I think we should call him Giampaolo." Anonymous texter
"It is a good thing this game has goals. If it was goaless I would have fallen asleep as the new seating is far too comfortable! William, Norfolk, via text on 81111
1411: Absolutely awful defending almost leads to a goal for Italy. The visiting team have really shown their class in the second half.
1408: It should be four for Giampaolo Pazzini after Lee Camp parries a shot from Derby's Arturo Lupoli but the Fiorentina striker cannot quite guide the ball home. Pazzini is substituted moments later and receives a generous standing ovation.
1406: It has been an afternoon to forget for Kieran Richardson. He has not been a regular for Manchester United this season and, though willing, hit a series of stray passes and poor shots. he is substituted.
1402: Italy have gone up a gear since equalising. Currently, they look the most likely to score. Former Chelsea striker Pierluigi Casiraghi - now the Italy Under-21 coach - has a cut a figure of sartorial elegance on the touchline throughout the match with his sharp suit and coat.
"I got a half-time cup of tea to warm myself up but have managed to spill it all over my crotch. Loving the stadium though!" Anonymous texter
"Giampaolo Pazzini's movement really is absolutely fantastic." Chris Waddle, BBC Sport
1356: GOAL England U21s 3-3 Italy U21s Giampaolo Pazzini - remember the name. The Italian has just become the first man to score a hat-trick at the new Wembley. He picks up a through ball on the right and shoots through a defender's legs from a tight angle. Lee Camp may feel that he should have done better. That said, some of the defenders in front of him might not want to see a replay either.
"There was water on the floor in the toilets.." Anonymous texter
1350: Kieran Richardson should extend England's lead but smashes his volley high and wide from a deep James Milner cross. After a relatively prosaic opening half, it has been all action since the break.
1347: GOAL England U21s 3-2 Italy U21s Matt Derbyshire puts England back in front, turning home a driven strike from James Milner. There is a suspicion of offside but the flag stays down. Boos turn to cheers.
1342: GOAL England U21s 2-2 Italy U21s
England fail to clear a corner and Giampaolo Pazzini guides Antonio Nocerino's cross-cum-shot into the roof of the empty net from five yards. Cheers have turned to boos inside the stadium.
1341: GOAL England U21s 2-1 Italy U21s Wayne Routledge, who has not scored in the Premiership this season, puts the home team in front. Giorgio Chiellini fails to cut out Matt Derbyshire's cross and Routledge smashes the ball home from a tight angle. Stuart Pearce, watching from the stands, is heard to shout "great ball" during the build-up.
1336: No news reaching me of any major access and facility problems at half-time. How have you found the Wembley experience? Let me know by texting 81111. That said, there was apparently a problem with the coffee machine in the press room!
1334: The match resumes, with England introducing Matt Derbyshire for Gabriel Agbonlahor.
"What a stadium! Believe it or not it runs Grimsby Town's Blundell Park very close." Anonymous texter
1318: Half-time England U21s 1-1 Italy U21s
The whistle sounds and the players make their way down the tunnel and into their respective dressing rooms. A patchy game on a pitch that seems to be cutting up quite badly. Safety certificate watch - everything seems to be passing off pretty well.
1314: David Bentley, who has been impressive, shoots wide. Giorgio Chiellini leaves the field with a gash in his head after Gary Cahill catches him and, seconds later, Gabriel Agbonlahor heads over the crossbar.
1311: Andrea Mantovani should score with a free header from a corner but directs the ball to close to keeper Lee Camp.
1310: Moving towards the half-time break. England have improved as the match has worn on, though after their awful start it could not have been much worse.
"Immense. When the goal went in the surge of emotion was incredible. Men all around me were in floods of tears." Anonymous texter
1308: Kieran Richardson shoots wide from distance. The England boys have not been shy at trying their luck from long range. Keen to get on the scoresheet?
"It is a decent free-kick but you have to question the goalkeeper." Chris Waddle, BBC Sport
1302: GOAL England U21s 1-1 Italy U21s David Bentley equalises for England with a free-kick that nestles in the bottom corner. A reward for the team's endeavour - and something for the crowd to really make some noise about.
1258: The quality of the Italian team in possession is superb. Another patient, precise move culminates in a cross that Alessandro Rosina heads narrowly over.
1255: The clever Giampaolo Pazzini loses Anton Ferdinand to collect a wonderful floated through ball. His shot is blocked but he is clearly a player of some ability. As for Anton...
1251: The England team are starting to impose themselves on the fixture but have been let down by a series of atrocious corners and crosses. Simply not good enough. Crowd watch - gone a bit quiet.
1247: Leroy Lita is played through and about to burst clear but is hacked to the floor by Andrea Mantovani. Kieran Richardson takes the subsequent free-kick but his strike hits the wall and flies over the crossbar.
1245: Italy keeper Gianluca Curci makes a timely interception, clearing the ball with Lita lurking.
1242: England win their second corner of the match. It comes to nothing. Italy look extremely comfortable in possession. Cannot say the same of their opponents.
1240: A blast of 'Eng-er-land" sounds as Italy's Simone Padoin shoots wide from distance.
1238: Stunned by the start to the match, the crowd have not exactly found their voice as yet, though a Mexican wave is doing the rounds. Don't need that, do we?
"The pitch is cutting up slightly and a bit bobbly. England need a spell of possession." Chris Waddle, BBC Sport
1231: GOAL England U21s 0-1 Italy U21s
Who will be the first man to score at Wembley? Italy's Giampaolo Pazzini after 25 seconds. His thunderous long-range strike flies past Lee Camp, taking a slight deflection off Anton Ferdinand on the way.
1230: The match starts, 2,358 days after the old Wembley closed its doors. England U21: Camp, Rosenior, Ferdinand, Cahill, Baines, Bentley, Reo-Coker, Richardson, Routledge, Agbonlahor, Lita.
Subs: Alnwick, Hoyte, Ridgewell, Cattermole, Milner, Huddlestone, Young, Jerome, Hart, Derbyshire. Italy U21: Curci, Potenza, Mantovani, Andreolli, Chiellini, Rosina, Montolivo, Nocerino, Pazzini, Rossi, Padoin.
Subs: Viviano, Coda, Lupoli, De Martino, Dessena, Raggi, Pelle, Criscito, Paonessa, Lazzari.
What a fantastic stadium! These seats are better than my living room chairs. The game should be immense. Kieran via text on 81111
1226: The two teams are out on the pitch and being introduced to various dignitaries. The national anthems follow.
1221: The minutes tick down to kick-off. Stuart Pearce is the head coach of the England Under-21s but he has been concentrating on his job as manager of Manchester City this week so the team will be led out by stand-in boss Nigel Pearson. With Man City not in action today, Pearce is watching the match from the stands.
"They need to sort out the programmes at Wembley. It is taking ages to get one." Steve via text on 81111
1215: With Preston's David Nugent out in Israel with the senior squad, Leroy Lita and Gabriel Agbonlahor will start up front. Will one of these two score the first goal at Wembley?
"The place (Wembley) is amazing, a fantastic stadium. I would love to play here. This is a special occasion and a special day." Former Chelsea striker Gianfranco Zola, now involved with the Italy Under-21 team.
1205: The England and Italy Under-21 players are going through their warm-up routines on the pitch. People are starting to take their seats. Lets not forget that for the England Under-21 team this game is a warm-up fixture ahead of this summer's European Championship finals in Holland.
"Just entered Wembley - awesome!! Cannot wait for the start, the atmosphere should be superb." Sam Wass and family via text on 81111
"The new Wembley looks more impressive as it starts to fill up. First impressions - a distant relative of the Stadium of Light in Lisbon, Portugal." Phil McNulty, BBC Sport
1155: People are pouring into Wembley now, though not all that many have taken their seats. The match is a 60,000 sell out - and is apparently set to be the highest attended Under-21 fixture ever. It is the second of two "ramp-up" events following last week's community day that involved 30,000 people.
1145: The fans are continuing to make their way into the stadium with kick-off now 45 minutes away. Rich Jones has just texted us via 81111 to say that he has been given a free scotch egg. Sounds good to me.
1130: The staff at Wembley and the Football Association will be desperately hoping that everything passes off smoothly - and that the stadium receives its General Safety Certificate from Brent Council. Without it...
"There have been rumours that there are forged tickets around but so far everything seems to be going very smoothly." Andy Swiss, BBC Sport
"The new Wembley looks great, but the tea machines are broken already." Andy Maxwell via text on 81111
1115: Lots of fans are stopping to take photos using their mobile phones as they emerge from Wembley Park Station. So many, in fact, that they are causing congestion and being told to move on. Unexpected teething trouble?
1100: Fans are steadily making their way down to Wembley. The clouds have closed in over the stadium after a sunny start to the day.
"The tubes are packed with England fans desperate to get an early taste of the new Wembley." Phil McNulty, BBC Sport
1045: Great news! 606 user cafasmokingnun has got his tickets and is inside Wembley. He received a text message informing him to pick them up at a hotel near the ground and they were ready and waiting when he arrived. No further reports of any ticket problems so far.
"Just remember Scotland have won a game at Wembley more recently than England. And we have our eye on these goal posts!" Colin, Johnnie and Ally (going to Bari) via text on 81111
1020: Don't forget live commentary of the game will be available on this website (UK users only). In the meantime, Will has texted asking us to make an emotional appeal to one of his mates.
"Could you tell Jimmy to shake off his hangover and get ready!" Will via text on 81111
"I'm very excited and looking forward to the game, though I'm not sure how much of it I will see." Alex Horne, managing director Wembley National Stadium Limited
1000: On Friday we received a 606 message from cafasmokingnun, who talked of his horror at the news that his tickets had not arrived. He has paid big money for him and his son to become Club Wembley members and was gutted that he was still ticketless less than 24 hours before kick-off. He tried telephoning and e-mailing without success - and was very unhappy. We'll try to find out whether he is able to get hold of his precious tickets.
"Just leaving for Wembley now. Can't wait. The highlight of a dismal week for me, with Spurs out of the FA Cup and the death of Bob Woolmer." Anonymous texter
0940: Don't forget - the kick-off time for the match is 1230 GMT. I wonder how many people have decided to make a real Wembley weekend of it by watching Lionel Richie in concert at Wembley Arena on Friday evening?
0930: The scheduled opening time for the turnstiles for Club Wembley members. They will open for ordinary ticket holders at 1030.
The 60,000 tickets for the match sold out in six hours and the fixture is the second of two events held at Wembley to ensure the stadium can receive its General Safety Certificate.
0908: A text reaches us saying quite simply "On the train to Wembley!" The first match at the rebuilt stadium is just hours away - and you can text us on 81111 (UK users only) or send videos and pictures to 60011. Let us know how you're travelling to the new stadium - and what you think of it when you get there.
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