Wayne Rooney inspired Manchester United as they broke Watford's brave challenge to reach the FA Cup final at Wembley.
Rooney powered home a brilliant 20-yard drive after only six minutes, but Watford levelled with Hameur Bouazza's 26th minute overhead kick.
Cristiano Ronaldo bundled in Rooney's cross two minutes later as United swiftly restored their lead.
Rooney's close-range finish wrapped up the win after 66 minutes and substitute Kieran Richardson chipped in a fourth.
The scoreline looks comprehensive, but Watford were well in contention until Rooney's crucial second and United's third.
The only low point for United was a first-half injury to Rio Ferdinand, who limped off with a groin problem.
United resisted the temptation to rest their stars after the spectacular Champions League win against Roma - and the move paid dividends as Rooney struck brilliantly after only six minutes.
Alan Smith dummied Michael Carrick's pass, and Rooney's first touch was instant before he lashed a rising 20-yard drive past Richard Lee.
United threatened to run away with the semi-final in the early stages, but Watford held on and were rewarded with a stunning equaliser after 26 minutes.
Edwin van der Sar had just received lengthy treatment for a nose injury, and he was still recovering when Tommy Smith headed into the path of Bouazza, who sent an overhead kick into the top corner via a deflection off Gabriel Heinze.
But Watford's celebrations were short-lived as United regained the lead two minutes later.
Rooney was the creator, racing clear and crossing for Ronaldo to bundle in from six yards.
Man Utd: Van der Sar, Evra, Ferdinand (Fletcher 40), Brown, Heinze, Ronaldo (Richardson 77), Scholes, Carrick, Giggs (Solskjaer 83), Smith, Rooney. Subs Not Used: Kuszczak, Cathcart.
Booked: Rooney, Carrick.
Goals: Rooney 7, Ronaldo 28, Rooney 66, Richardson 82.
Att: 37,425. United suffered a blow six minutes before half-time, when Ferdinand was forced off with a groin injury and was replaced by Darren Fletcher.
Watford keeper Lee then kept his side in the game with a fine one-handed save from Rooney after a slick exchange with Ryan Giggs.
Fletcher then headed inches wide as United ended the first half in the ascendancy.
Watford, backed by thousands of fans bedecked in yellow, started the second period with real vigour, and Bouazza turned a close-range effort agonisingly wide as the Hornets threatened an equaliser.
But it was effectively all over after 66 minutes, and Rooney was once again United's main man.
Slack marking by Watford allowed Smith too much space, and he cleverly picked out Rooney for a simple finish.
Watford were in danger of being swamped by United, and Lee needed to be alert again to deny Giggs.
There was one bonus for Watford late on when striker Marlon King, who has been out for virtually the whole season with knee injury, came on as a substitute to a rousing ovation.
Ronaldo was rested to give Richardson a run-out, and he made the most of it by running on to another superb pass from Smith to lift a finish over Lee with eight minutes left.
Fund of semi-final memories like no other ground.
The penultimate round of the FA Cup has been stayed.
This time Manchester United have been in an FA Cup.
As for Watford, well their fans have come with their colourful.
Those who lost relatives.
You can go at noon and buy a ticket.
Suspension, Alan Smith continues up front with Wayne Rooney.
Manchester United have the best ever semi-final goals.
Turning Paul Sculls.
He's in his home city of Birmingham.
They just replaced O'Shea with.
Is now over on this great day of sport.
Rooney.
A little tumble by him.
Free kick says Howard Webb.
Haintzer.
Smith's coming in on the cross.
It'll be a good early touch.
Giggs let that run.
That was Smith nearly.
And it's Rooney on the ball now.
And Rooney will shoot!
Oh!
What a start for Manchester United.
And who else but Wayne Rooney.
Six minutes gone.
What a decisive finish.
Well, a couple of dummies there by Manchester United players, Mark.
And when Rooney...
He might fancy a crack himself, but he's fouled Giggs.
And there's Rooney again, this time with a header.
17 times.
Oh, it's a corner.
17 times in Ligue 1 FA Cup.
Mane and Francis Chambers in the middle.
Both of them are there.
Headed by...
Foul on Tommy Smith, isn't it?
Howard Webb says no.
Ronaldo.
Smith.
Rooney's coming in across the box now.
It's a good early ball as well.
I thought he got the ball cleanly there, Alan Smith.
It's something excellent he gives you as a striker.
Rooney's coming in from this side against Mariapa.
And here's Smith!
What a strike that was.
That bounced up so nicely for him as well.
Great leap by Rooney.
Oh, look at the way Scholes shook off...
Wow.
The first foul is Scholes on Mane.
And then Scholes was brought down as well.
Demeritz with the flick on.
A well-rehearsed Watford move there.
I think the last touch came off Rio Ferdinand,
who didn't really know too much about...
Where's this going to finish up?
Bouazza with the shot.
Up goes Van der Sar.
Carrick lets it go to Scholes.
It's got Priskin inside him.
Francis has also gone into the box now.
It's Bouazza's swerve.
Good with his left foot.
Got the winner at Plymouth in the sixth round.
And two on Mane.
Here comes Carlyle again.
That was won by Ferdinand.
This is Tommy Smith.
Lobbing it back in.
Oh, and a shot on the turn!
And a goal!
And it's Bouazza.
What a goal by Hammur Bouazza.
Out of nothing, he has equalised for Watford,
and their section of the ground has gone crazy.
Van der Sar was rattled by the injury,
and he was beaten by the next effort,
and Bouazza proves again.
Oh, and Smith's there.
Could have been 2-1.
Great tackle by Clark Carlyle.
Great tackle.
Again now.
This is dangerous, it's Wayne Rooney.
Is he going to go himself?
Tried to, Ronaldo!
It's 2-1.
Well, a little bit of luck maybe,
in that Rooney tried to score,
but Alex Ferguson is off the bench
and Manchester United have regained the lead
within two minutes.
John, the timing of the pass from Smith
and the run from Rooney are crucial.
Just waited in.
Rooney was away completely, loads of time.
And it's Ronaldo, he's through again.
Demeric...
This is Mane.
He does strike this one.
Oh, and Van der Sar has done the same again.
Turned it out, but it could have gone to a Watford player.
It could, I think it gets a deflection as well.
Could take this free kick.
Oh, it was Francis coming to the near post.
Just warned him, Johnny,
probably got a little bit too much on it.
Giggs let that run, this is Smith.
Rooney, Giggs, Rooney again!
What a good save by Richard Lee
to prevent Manchester United going 3-1 ahead.
Well, this is brilliant.
That's Fletcher.
Just drops away.
Goal kick.
Well, how lucky here are Watford.
Nobody on...
Flag going up against Rooney.
No, it's not, he's onside.
And he's looking menacing here,
his way, Rooney!
Oh, a little chipped attempt there.
Wouldn't put it past him to do that again
as he did in one of the earlier rounds.
He's trying to dink it, isn't he?
Up goes Carlyle.
Where's it going to drop?
Carlyle again, Demeric nearly got in.
And once more, Carlyle.
Well, that would have been some goal, wouldn't it?
Played here down the years.
Smith makes the run.
Fletcher is his opponent here.
Finds Priskin.
Bouatsa wants it laid into the left.
That's his shot.
Good football.
That's Carlyle.
Oh, it wasn't well taken there.
And Bou...
Oh!
Well, Bouatsa might have repeated his trick
of the first half there.
It came off Ainsley, I think.
This was Watford.
It isn't a bad turn by Priskin here.
This is Bouatsa.
And he's away!
He's passed Ainsley.
And Damian Francis is in a great position.
But he couldn't find him.
And it came off Edward for a corner.
Well, two things, Jon.
He should do for Watford.
There are five in the middle.
Ainsley is down this right-hand side.
Giggs is coming in now.
It's gone right across.
Gingham up into the centre now.
There he is, Francis.
And it's come out to Marne.
They're saying,
Shoot, the Watford fans.
Well, that's his second effort of the day, isn't it?
It was a strange piece of play by Anders here.
Good challenge by Jordan Stewart.
Oh, but this is Alan Smith across the goal!
Oh, and Rooney was there!
And it's gone in.
Wayne Rooney, 3-1.
But he made a lot of the running himself in that move.
And he may have scored the goal that sends Manchester United to Wembley mark.
Well, I just think they're very, very slow here.
Watford to close him down.
It's a good tackle, an original good tackle.
And Chambers in support there as well.
And this could give Manchester United a lot of...
...ball by Jordan Stewart.
Priskin is there.
So was Francis.
The flag's up here.
I think it may be an offside against Priskin in the end.
Yeah, it is.
Confusion yet again.
A little touch by Giggs.
Oh, Alan Smith.
Surely Alan Smith.
Or Rooney for the hat-trick.
Not quite.
Well done the goalkeeper.
He read exactly what Rooney was going to do.
He was just trying to pass it past him.
That's Cabot.
Good cross.
Oh!
Damien from the penalty area.
And it's still Ryan Giggs.
Alan Smith slides the ball in.
Oh, and Kieran Richardson made a run there.
And Manchester United have gone 4-1 up.
And Kieran Richardson, only a couple of minutes after coming on,
has scored the fourth goal in the semi-final.
It's a cheap goal to concede from Watford's point of view, I'm afraid.
Smith as well, under no pressure.
Waits for the run from Richardson.
Richardson himself thought it was.
Alan Smith.
Rooney on this side.
Manchester United could slide Rooney in here.
Possibly for the hat-trick.
No.
Now, good challenge by Demerit, wasn't it?
Demerit slide.
Solskjaer picks out Alan Smith and Rooney.
But just at crucial moments of this game, really, they've gone to sleep.
For Solskjaer, maybe.
And Michael Carrick followed up.
The new Wembley.
Sir Alex Ferguson has taken them into the fairly comfortable.
From Rooney they went 2-1 up through Rooney.
Manchester United 4.
