Manchester United marked Sir Alex Ferguson's 20 years as manager by cruising to victory - and extending their Premiership lead to three points.
Saha (right) scored after three minutes
Louis Saha opened the scoring from the penalty spot after Portsmouth's Dejan Stefanovic tripped Wayne Rooney.
Cristiano Ronaldo then scored a superb free-kick, while David James denied Gary Neville three times in the match.
Niko Kranjcar missed a great chance for Pompey before Nemanja Vidic headed his first goal at Old Trafford.
Ferguson celebrates two decades at the helm on Monday - and must be extremely satisfied with the calibre and form of his current crop of players.
United have not lost in the Premiership since defeat by Arsenal on 17 September, though Chelsea can pull level on points with them at the top of the table if they beat Tottenham at White Hart Lane on Sunday.
The home team eased to victory and if Ferguson's men could be faulted it was that they should have scored more goals, though Pompey keeper James made a series of excellent saves.
The match was effectively over as a contest when Ronaldo scored United's second goal after 10 minutes.
Saha had already put United ahead, his penalty just eluding James' valiant dive, before Ronaldo made the most of Ryan Giggs' intelligent play.
The Wales winger lined up at the edge of the Pompey wall after a free-kick had been awarded for a poor tackle on Saha by Andy O'Brien.
Giggs peeled away as Ronaldo approached the ball and the Portugal star smashed his free-kick through the gap and into the net.
United, with such a commanding advantage so early in the match, launched attack after attack upon their bewildered opponents.
Neville twice came close to a rare goal for United in the first half but each time was thwarted by James saving with his legs.
James was having an extremely busy afternoon and after his block broke to Paul Scholes he had team-mate O'Brien to thank for heading the United midfielder's shot clear of goal.
Rooney was in fine form; the catalyst for numerous United attacks with his precise, clever kicks and forceful dribbling.
Pompey, in stark contrast, mustered nothing of worth until the stroke of half-time when Edwin van der Sar made a superb save to keep out Matthew Taylor's thunderous long-range strike.
Rooney almost scored straight after the restart with a low strike from distance - while at the other end Kranjcar wasted an excellent chance, missing the target after the ball broke to him in space 12 yards from goal.
United continued to press and Rooney shot wide under pressure following a mazy run before Vidic added a third from Neville's cross.
James made further saves from Neville and Saha, while substitute Darren Fletcher shot wide.
Pompey never stopped trying - Van der Sar saving a Gary O'Neil free-kick - but United were comfortable winners.
Manchester United manager Sir Alex Ferguson:
"We have had a fantastic start (to the season). Historically, we don't always have good starts.
"We have made one this year and we need to take advantage of it.
"I have to be honest and say there is a confidence about the place that suggests we are now in a position to pull out all the stops and go for it."
Portsmouth assistant manager Joe Jordan:
"I am sure they are capable of giving the reigning champions a real run for their money.
"They still have to prove they are capable of maintaining that form throughout the season.
"But at this moment in time, there is no question they are flying. They are playing with great confidence."
Man Utd: Van der Sar, Neville, Ferdinand, Vidic, Evra,
Ronaldo (Fletcher 75), Carrick (Silvestre 71), Scholes, Giggs,
Rooney (O'Shea 75), Saha.
Subs Not Used: Kuszczak, Brown.
Goals: Saha 3 pen, Ronaldo 10, Vidic 66.
Portsmouth: James, Pamarot, O'Brien, Stefanovic, Taylor,
Pedro Mendes, Davis, Kranjcar (Hughes 84), Fernandes (Cole 46),
O'Neil, Mwaruwari (LuaLua 66).
Subs Not Used: Kiely, Kanu.
Ref: M Dean (Wirral).