Ljungberg strike puts Arsenal into final
But it needed a brilliant save from England goalkeeper David Seaman to deny First Division United an equaliser six minutes from time.
Arsenal scored the decisive goal in the 34th minute but only after England centre-back Sol Campbell appeared to foul United striker Wayne Allison from behind.
But, as Allison lay on the ground, referee Graham Poll allowed play to go on.
And then, as the ball was advanced upfield, Poll collided with Michael Tonge as the United midfielder went to intercept.
But instead the ball came to Arsenal striker Francis Jeffers out on the left and his pull back found Sylvain Wiltord. His shot came back off the post.
However, the France forward followed up his own strike and as the ball was cleared off the line by United captain Robert Page, it fell kindly to Ljungberg who blasted his shot into the roof of the net.
Angry United players surrounded referee Poll but the goal stood and at half-time Blades manager Neil Warnock also confronted the official.
It was the second time in recent seasons that Arsenal had scored a controversial goal against United in a Cup tie.
Back in 1999 Arsenal won a fifth round tie at Highbury 2-1 when, after Nwankwo Kanu refused to return the ball after United had put it out so an injured player could receive treatement, Marc Overmars scored.
Such was the furore surrounding the goal that Arsenal immediately offered to replay the tie and they won the second match 2-1.
It was a harsh way for United - who reached the semi-finals of the League Cup earlier this season - to concede after they had harried and hassled the Premier League champions out of their stride.
Promotion chasers United might have had an equaliser after 52 minutes when Michael Brown’s precise pass found Peter Ndlovu unmarked inside the box.
But his shot was straight at Seaman who saved easily.
But Seaman, 39, produced a magnificent stop six minutes from time to deny United what seemed a certain levelller.
Following a corner, Carl Asaba’s volley came to substitute Paul Peschisolido inside the six-yard box.
After 56 minutes Arsenal lost captain Patrick Vieira who limped off with what appeared to be a recurrence of his knee injury.
FA Cup semi-final results
Results:
Arsenal 1 Sheffield United 0
Watford 1 Southampton 2
SPANISH FIRST DIVISION
Atletico Madrid 3 Espanyol 3
Celta Vigo 1 Real Betis 0
Malaga 1 Valladolid 0
Rayo Vallecano 1 Athletic Bilbao 1
Real Mallorca 3 Racing Santander 3
Recreativo Huelva 1 Alaves 0
Played Saturday:
Barcelona 2 Deportivo Coruna 4
Sevilla 3 Villarreal 1
ITALIAN FIRST DIVISION RESULTS
Atalanta 1 Reggina 1
Empoli 2 Chievo Verona 1
Modena 0 Lazio 0
AS Roma 2 Parma 1
Torino 1 Piacenza 3
Udinese 0 Brescia 0
Played on Saturday:
Como 1 Perugia 1
Inter Milan 0 AC Milan 1
GERMAN FIRST DIVISION RESULTS
Schalke 04 3 Energie Cottbus 0
VfL Wolfsburg 1 Hanover 96 0
Played on Saturday:
Borussia Moenchengladbach 2 Nuremberg 0
Arminia Bielefeld 2 TSV 1860 Munich 1
Bayern Munich 0 Werder Bremen 1
Kaiserslautern 1 Hansa Rostock 0
Hertha Berlin 1 VfL Bochum 0
VfB Stuttgart 3 Bayer Leverkusen 0
Hamburg SV 1 Borussia Dortmund 1
—AFP/Reuters