Rooney's header set United on their way to a comprehensive win
Wayne Rooney met Darren Fletcher's cross to head the opener and the lead was doubled when Nani's low cross was deflected in by Anthony Vanden Borre.
Michael Carrick's drive hit Richard Hughes and thumped in off the bar and Dimitar Berbatov's shot made it 4-0.
Pompey's misery was complete when Marc Wilson tried to clear a cross but smashed a volley into his own goal.
It was a ruthless performance from United but the reality was they barely had to break sweat to beat a Pompey side in freefall.
Phelan encouraged by Man Utd's quality
United went in to the game knowing a win would see them leapfrog Chelsea to sit at the Premier League summit, with the Blues hosting third-placed Arsenal on Sunday.
And on paper it seemed they could not get an easier opportunity, with crisis club Pompey having won only once in eight matches, as well as attracting unwanted headlines for their financial difficulties and allegations about their manager Avram Grant's behaviour.
United's off-the-field problems were of the more traditional kind - Rio Ferdinand was out suspended and fellow centre-back Nemanja Vidic was still not fit to return to action.
Not that United's backline needed to be at its strongest, with Pompey chasing shadows for much of the game as Nani, Antonio Valencia and Rooney ran riot.
Predictably, the visitors' defence folded under pressure, with Pompey contributing two of United's goals.
Grant's men did well to reach half-time having conceded only two goals as they found themselves struggling to repel wave after wave of United attacks.
Rooney had a decent penalty claim turned down after being pushed by Frederic Piquionne, while Berbatov missed a sitter from six yards after being picked out by Gary Neville.
Pompey did manage to carve out one chance and it took an alert Jonny Evans to clear Jamie O'Hara's goal-bound effort.
But it was a very rare effort from Pompey and they soon went behind.
When Fletcher's cross was left, inexplicably, by keeper David James, Rooney headed in from close range.
James was again left cursing soon after when he was wrong-footed by a deflection off Vanden Borre from Nani's cross.
Grant committed to Pompey cause
Belgian defender Vanden Borre tried to make amends with a decent shot which was well saved by Edwin Van der Sar, who also smothered an effort from Nadir Belhadj.
That proved the last of Pompey's forward play - in fact they then firmly hit the self-destruct button.
They were unfortunate when Carrick's speculative drive cannoned off Hughes and beat James, but they had only themselves to blame for United's fourth goal.
Berbatov was allowed to twist and turn in the area before taking the ball back outside the box and shooting into the bottom corner from acres of space.
It was abysmal defending from Pompey - and worse was to follow.
With no-one around him, Wilson's attempt to cut out Patrice Evra's cross ended with him sending a rasping volley past the hapless James.
United, who had long since taken the chance to rest the brilliant Rooney, should have had a sixth.
Substitute Mame Biram Diouf played the ball one side of Tal Ben-Haim and collected it round the other but then blazed his shot over.
Man Utd boss Sir Alex Ferguson:
"We had a lot of pressure and possession of the ball but we were patient enough.
"That's important when teams come and sit in the way they did here today. Portsmouth were there to make it difficult but we got the goal just before half-time.
"We were a bit fortunate with the second one. Nani beat his man and was trying to play a cross across the box.
"It got a deflection and went past James and into the net. It was fortunate but nonetheless you have to take them. That opened the game for us in the second half.
"We went looking for goals and managed to get three more, the one from Berbatov was brilliant."
Portsmouth manager Avram Grant:
"Positive thinking is always better than negative thinking and as long as we have the chance we will fight.
"We created two good chances and one came off the line. Then after 40 minutes they scored and got some own goals but Manchester United are a better team than us.
"We need to do our job even if it feels like things are going against us. We cannot think like this."
From BBC
In short, Proactive simply seeks the monies to which it is contractually entitled 
Posts
