Match ends, West Ham United 2, Watford 4.
Watford produced a stunning performance to come from 2-0 down to beat West Ham.
Two headers from Michail Antonio had put the hosts firmly in control with 33 minutes played.
However, Odion Ighalo's deflected shot and a wonderful effort from Troy Deeney levelled it before the break.
Etienne Capoue's powerful half-volley put the visitors in front and Jose Holebas sealed Watford's first victory of the season with a shot from 18 yards out.
The comeback of the season?
There had been little indication of what was to follow when Antonio headed home Dimitri Payet's rabona cross with just over half an hour played.
The Hornets were utterly outplayed in the opening exchanges of this game and were lucky not to be further behind, Daryl Janmaat inexplicably passing the ball against his own post when under no pressure.
Ighalo got the ball rolling for his side when he drifted inside from the left and his low shot went in after a kind deflection from James Collins.
Collins' afternoon went from bad to worse just before the break when a breakdown in communication saw the Wales international head the ball over the onrushing Adrian and Deeney picked up the loose ball before curling home a superb shot from just inside the area.
The Hammers had half-time to regroup but simply could not match Watford's intensity in the second half.
Capoue scored his third goal of the season when a deep ball came to him at the back post and Holebas grabbed his first goal in English football with a shot from the edge of the area that Adrian should have done more to keep out.
From there on in the Hornets had further chances to add to their score but Adrian kept out efforts from Isaac Success and Roberto Pereyra.
West Ham's worrying capitulation
It has not been a great start to the season for the Hammers.
Slaven Bilic's side were knocked out of the Europa League by Romanian side Astra Giurgiu for the second season running and they have now lost three of their first four Premier League matches.
Bilic gave a first start of the season to Payet and for the first 30 minutes the Frenchman was absolutely magnificent.
His corner was headed in by Antonio as the hosts took an early lead and he then set up the second with possibly the best pass of the campaign so far.
He turned Holebas inside out before stopping and sending over a rabona cross with his right foot that Antonio converted for a fourth league goal of the season.
After that Payet faded as his team totally lost control of the match.
Like many of his team-mates debutant Simone Zaza started brightly, releasing Antonio with an overhead kick with just his second touch in English football, but by the time he was subbed off on 77 minutes he had become an isolated figure.
Hammers striker Andy Carroll is still a couple of weeks away from returning and, on this evidence, they desperately need his presence to bolster their attack.
Analysis - comical defending
BBC pundit Alan Shearer on Match of the Day
Taking nothing away from Watford, who showed great character to come back, but the defending was comical.
For the second goal, it was laughable defending. West Ham were all over the place. Watford were superb in coming back though.
West Ham to review stadium security
West Ham have pledged a "full review" of security at the London Stadium after fighting broke out during the match.
Trouble flared between rival supporters and the club later released a statement citing "zero tolerance" on fan violence. It said offenders would be "banned for life from any West Ham fixture".
The stats you need to know
- Today was the first time Watford have won a Premier League game having been 2+ goals behind (L37 before today).
- West Ham have lost two of their last four home Premier League matches after going 15 without defeat beforehand.
- Watford have scored four goals in a PL away game for the first time.
- Michail Antonio has scored a league-high 10 headed goals since the start of last season.
- Since their return to the Premier League, Odion Ighalo (16) and Troy Deeney (14) have scored almost two-thirds (64%) of Watford's 47 Premier League goals.
- Etienne Capoue has scored three goals in four appearances this season after netting just one in his previous 57 in the Premier League.
What the managers said
West Ham boss Slaven Bilic: "It was a shock to be 2-2 at half-time but there were still 45 minutes to go.
"We let them score two goals in the second half without putting them under pressure. It was too easy for them.
"We can not defend like that or we are not going to win a single game.
Watford boss Walter Mazzarri said: "We didn't start very well but we started to concentrate better and the result came.
"We changed a couple of things in our formation at half-time and that allowed us to not risk as much.
"It was a great win but I look at the way we played and we have to correct the mistakes we made.
"This win gives us the confidence to know we are a very good team and can go on to have a very good season."
Man of the match - Roberto Pereyra
West Ham visit West Brom on Saturday while Watford host Jose Mourinho's Manchester United on Sunday lunchtime.
- 22ByramBooked at 84minsSubstituted forTöreat 85'minutes
- 16NobleSubstituted forCalleriat 69'minutes
- 11ZazaBooked at 12minsSubstituted forFletcherat 77'minutes
- 15CathcartBooked at 42mins
- 4KaboulSubstituted forPrödlat 82'minutes
- 9DeeneySubstituted forOkakaat 78'minutesBooked at 86mins
- 24IghaloSubstituted forSuccessat 69'minutes
- Martin Atkinson
- Shots on Target
Second Half ends, West Ham United 2, Watford 4.
Attempt missed. Cheikhou Kouyaté (West Ham United) header from the centre of the box is just a bit too high. Assisted by Manuel Lanzini with a cross.
Foul by Michail Antonio (West Ham United).
Isaac Success (Watford) wins a free kick on the left wing.
Offside, West Ham United. Dimitri Payet tries a through ball, but Winston Reid is caught offside.
Corner, West Ham United. Conceded by José Holebas.
Foul by Winston Reid (West Ham United).
Stefano Okaka (Watford) wins a free kick on the right wing.
Stefano Okaka (Watford) is shown the yellow card.
(West Ham United) wins a free kick on the left wing.
Stefano Okaka (Watford).
Winston Reid (West Ham United) wins a free kick in the defensive half.
Foul by Stefano Okaka (Watford).
Substitution, West Ham United. Gökhan Töre replaces Sam Byram.
Sam Byram (West Ham United) is shown the yellow card for a bad foul.
Foul by Sam Byram (West Ham United).
Isaac Success (Watford) wins a free kick in the defensive half.
Corner, West Ham United. Conceded by Daryl Janmaat.
Attempt saved. Stefano Okaka (Watford) right footed shot from the right side of the box is saved in the top centre of the goal. Assisted by Isaac Success.