Match ends, Arsenal 2, Manchester United 0.
Ole Gunnar Solskjaer suffered his first Premier League defeat since being appointed Manchester United's interim manager as Arsenal leapfrogged their opponents to move into the top four.
Switzerland midfielder Granit Xhaka beat David de Gea with a swerving shot from outside the penalty area, which the United keeper was expected to stop.
Romelu Lukaku hit the bar while the game was goalless, when it appeared easier to score, and, soon after Arsenal took the lead, Brazilian midfielder Fred struck a post for United.
Arsenal keeper Bernd Leno also produced two crucial saves to keep out Lukaku, before Pierre-Emerick Aubameyang doubled the hosts' lead from the penalty spot after Fred's foul on Alexandre Lacazette.
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- Man Utd's Smalling shoved by fan
- Reaction from Emirates Stadium
- Arsenal loss lowest moment - Solskjaer
Soon after Arsenal's second goal, a fan ran on to the pitch and shoved United defender Chris Smalling before being removed by stewards.
The Gunners have won seven out of 10 top-flight games in 2019 and are one point behind north London rivals Tottenham, who sit third in the table, with eight games remaining.
United, who started the weekend in fourth, drop to fifth in the race for a Champions League spot next season.
Arsenal turn on the style to sink United
"Tottenham Hotspur, we're coming for you" chants filled Emirates Stadium at the final whistle as Arsenal fans celebrated a crucial win.
Last weekend, Aubameyang left Wembley dejected after his last-minute penalty was saved by Spurs keeper Hugo Lloris to deny Arsenal victory in the North London derby.
Against United, he stepped forward and beat De Gea from the spot to seal a first victory over the Red Devils in five league and cup meetings.
It was awarded after Fred's forearm nudged into Lacazette, sending the French forward tumbling inside the box.
Lacazette inspired one of Arsenal's finest days under Unai Emery's command.
The Gunners were sharp, lively and industrious as they made amends after United eliminated them from the FA Cup six weeks ago.
Emery named six players who started Thursday's Europa League defeat by Rennes in his starting line-up, but there was little sign of fatigue as the hosts came flying out of traps.
They opened the scoring with their only shot on target in the first half, Xhaka deceiving De Gea with a shot that left the United keeper rooted to his line.
Leno was also key for Gunners.
The German did well to frustrate Lukaku as the Belgium forward threatened to go around and equalise, before denying the same player with an outstretched leg after half-time.
Arsenal have now played all their Champions League chasing rivals - a place in Europe's most prestigious competition next season is very much in their own hands.
|Race for the Premier League top four|
|17 March Everton (A)|
|31 March Cardiff (A)||30 March Watford (H)||31 March Liverpool (A)|
|1 April Newcastle (H)||2 April Wolves (A)|
|7 April Everton (A)||8 April West Ham (H)||7 April Brighton (H)|
|15 April Watford (A)||14 April Liverpool (A)||13 April West Ham (H)||13 April Huddersfield (H)|
|20 April Crystal Palace (H)||22 April Burnley (H)||21 April Everton (A)||20 April Manchester City (A)|
|24 April Man City (H)|
|29 April Leicester (A)||28 April Man Utd (A)||28 April Chelsea (H)||27 April West Ham (H)|
|4 May Brighton (H)||4 May Watford (H)||4 May Huddersfield (A)||4 May Bournemouth (A)|
|12 May Burnley (A)||12 May Leicester (A)||12 May Cardiff (H)||12 May Everton (H)|
|Wolves (A) Date TBC||Brighton (H) Date TBC||Crystal Palace (H) Date TBC|
How will United respond?
United have got used to winning under Solskjaer and it will be interesting to see how they respond to a first domestic setback under the Norwegian.
After their incredible midweek exploits against Paris St-Germain, they started slowly - although Lukaku will wonder how he did not put United ahead from their first serious attack.
The visitors paid the price when they conceded their earliest league goal under Solskjaer and, despite creating chances, were not clinical enough in the final third.
Paul Pogba was disappointing on his return to the side but he was by no means the only United player who failed to reach recent heights.
This was only United's second defeat in 18 league and cup games under Solskjaer.
A place in the top four remains in their sight, although they will need an immediate reaction when they return to league action after this month's international break.
Man of the match - Alexandre Lacazette (Arsenal)
Arsenal's 'big six' improvement - the stats
- Arsenal have won 12 points from 10 games against fellow 'big six' sides in the Premier League this season, double the amount they won in such games last term.
- Manchester United lost away from home for the first time since 16 December.
- Six of Granit Xhaka's seven Premier League goals for Arsenal have come from outside the box.
- United failed to score for the first time in 22 away games in all competitions this season, last failing to find the net on the road in May 2018 at West Ham.
- David de Gea has made two errors leading to a goal in the Premier League this season - both of them against Arsenal.
Arsenal will attempt to overturn a 3-1 deficit when they entertain Ligue 1 side Rennes in the Europa League last-16 on Thursday (20:00 GMT). United are in FA Cup quarter-final action next Saturday at Wolves (19:55), a match which is live on BBC One.
- 5PapastathopoulosBooked at 38mins
- 31KolasinacBooked at 90mins
- 10ÖzilSubstituted forIwobiat 77'minutes
- 9LacazetteSubstituted forNketiahat 86'minutes
- 14AubameyangSubstituted forD Suárezat 80'minutes
- 22D Suárez
- 1De Gea
- 20DalotSubstituted forMartialat 71'minutes
- 31MaticBooked at 68minsSubstituted forGreenwoodat 80'minutes
- 6PogbaBooked at 76mins
- Jonathan Moss
- Shots on Target
Second Half ends, Arsenal 2, Manchester United 0.
Attempt saved. Marcus Rashford (Manchester United) header from the centre of the box is saved in the centre of the goal. Assisted by Ashley Young with a cross.
Sead Kolasinac (Arsenal) is shown the yellow card for a bad foul.
Mason Greenwood (Manchester United) wins a free kick in the defensive half.
Foul by Sead Kolasinac (Arsenal).
Foul by Luke Shaw (Manchester United).
Eddie Nketiah (Arsenal) wins a free kick on the right wing.
Substitution, Arsenal. Eddie Nketiah replaces Alexandre Lacazette.
Foul by Luke Shaw (Manchester United).
Denis Suárez (Arsenal) wins a free kick in the defensive half.
Foul by Ashley Young (Manchester United).
Alex Iwobi (Arsenal) wins a free kick on the left wing.
Substitution, Arsenal. Denis Suárez replaces Pierre-Emerick Aubameyang.
Substitution, Manchester United. Mason Greenwood replaces Nemanja Matic.
Foul by Fred (Manchester United).
Sead Kolasinac (Arsenal) wins a free kick on the left wing.
Offside, Manchester United. Luke Shaw tries a through ball, but Chris Smalling is caught offside.
Luke Shaw (Manchester United) wins a free kick in the attacking half.
Foul by Pierre-Emerick Aubameyang (Arsenal).