Halsall wins England's first gold in 50m butterfly (UK users only)
Fran Halsall claimed England's first gold medal of the 2010 Commonwealth Games by just three hundredths of a second in the women's 50m butterfly.
Halsall won gold in a British record 26.24 seconds, ahead of Australian world champion Marieke Guehrer.
England's Liam Tancock then cruised to men's 50m backstroke gold in a Games record time of 24.62 seconds.
Robbie Renwick won a first gold medal for Scotland in the last final of the day, the men's 200m freestyle.
Kate Haywood took bronze for England in the women's 50m breaststroke, while Stephanie Millward added another English bronze in the Para sport 50m freestyle S9, won by South Africa's Natalie du Toit.
However, England's Lizzie Simmonds surprisingly failed to reach the final of the 100m backstroke.
"It's a great start to the meet for us, a couple of golds," said the victorious Tancock, who holds the 50m backstroke world title.
Tancock enjoys 50m backstroke gold (UK users only)
"I had a text from Matt Clay, who won it four years ago in Melbourne, saying good luck - that was really nice and it's a special night.
"It felt pretty good but it didn't feel amazing. I gave it my best, though, and stroked on through to the win."
Despite his modesty about the performance, Tancock won by some distance, roaring into the lead and never facing much of a challenge from eventual silver medallist Hayden Stoeckel of Australia.
The Devon swimmer will compete again in the 100m backstroke event, later in the week.
Halsall helped herself to an excellent start in her 50m butterfly final, but only just held on for gold ahead of Australian duo Guehrer and Emily Seebohm.
The three swimmers on the podium were separated by just five hundredths of a second.
The 50m fly is not considered one of the Southport star's strongest events, and defeating both Guehrer and Seebohm is a surprise result for the 20-year-old.
Halsall said: "When I saw I'd won I thought, 'That was cheeky'.
"I enjoy being the underdog and when there are people better than me to take down, that's incentive to take it one step further.
"I changed my start a bit tonight, got up a bit earlier, got it right, did a good finish and it just happened."
"Fran doesn't get over-excited, she's very, very laidback," former men's 50m butterfly world champion Mark Foster told BBC Sport.
England's Ellen Gandy finished fifth in the final, ahead of Jemma Lowe of Wales in sixth.
Renwick wins first Scottish gold in Delhi (UK users only)
Renwick had looked off the pace for much of his 200m freestyle final, which was dominated for large stretches by fellow Scot David Carry and England's Ross Davenport.
But Glasgow swimmer Renwick, 22, came back to win in a time of 1:47.88, ahead of Australia's Kenrick Monk and Thomas Fraser-Holmes.
"I tried to stay relaxed, I knew Kenrick was coming back at me. This feels absolutely amazing right now," said Renwick.
"The 400m freestyle yesterday was a huge disappointment for me, I needed to turn it around."
Davenport eventually finished fifth, with another Englishman, Rob Bale, in seventh and Carry eighth.
Success for Australia came with a one-two in the women's 50m breaststroke final, Leiston Pickett winning in 30.84 ahead of Leisel Jones in 31.10, with Haywood a further seven hundredths of a second behind.
"I'm really, really chuffed. It's a bit of a bonus year, I wasn't expecting to do anything, so to do anything here is a bit shocking," said the 23-year-old.
Achieng Ajulu-Bushell finished fifth in the same final for England, with Scotland's Kathryn Johnstone seventh.
In the Para sport 50m freestyle S9 it was Du Toit, who competed at the Beijing Olympics alongside able-bodied swimmers in the 10km open water event, who won gold in a time of 29.17 seconds.
But Millward did enough to seal a bronze medal behind Australia's Annabelle Williams.
"I expected more but that start was a little quick, nobody would have been ready for that," said Millward, from Wiltshire.
Australia's Huegill has returned to swimming after retiring in 2004
Australia's Geoff Huegill, who won the Commonwealth men's 50m butterfly title in 2002 and is completing a return to the sport, progressed to the 50m fly final from Tuesday's semi-finals, as did England's Antony James.
Huegill, 31, retired after the 2004 Athens Olympics, but has lost around seven stone since coming out of retirement in 2008.
Brenton Rickard, Australia's 100m breaststroke world champion, looked strangely off the pace as he narrowly crept through his semi-final.
Rickard qualified seventh-fastest for the final, with Scottish duo Michael Jamieson and Kris Gilchrist through in fourth and sixth respectively, and Daniel Sliwinski of England rounding off the qualifiers in eighth.
England's world champion and world record holder Gemma Spofforth progressed from Tuesday's semi-finals of the 100m backstroke, but neither she nor Simmonds looked on top form.
Spofforth, who beat Simmonds to the European title, qualified as the sixth-fastest swimmer, with Georgia Davies of Wales through in eighth, but Simmonds missed out.
"I'm really disappointed with that swim," said the 19-year-old. "I'm going to go back and look at what went wrong. It's a big disappointment at a big meet."
Spofforth added: "I looked up at the scoreboard at the end and wasn't expecting to see those people going that fast. It's not as easy as I thought it was going to be."
Halsall also swam in the 100m freestyle semi-finals, finishing second in her race behind Australia's Alicia Coutts.
Three English swimmers made it through to the final - Halsall, Amy Smith and Emma Saunders. Saunders qualified eighth-fastest, by the margin of one hundredth of a second.