Match ends, Arsenal 3, West Ham United 1.
Arsenal head coach Unai Emery says his side "need to improve" in transition after the unconvincing Gunners recorded their first win under the Spaniard with victory over West Ham at Emirates Stadium.
The defensively frail hosts came from behind to lead in fortuitous fashion on 70 minutes as Hammers debutant Issa Diop diverted Alexandre Lacazette's volley across the area into his own net.
Hector Bellerin then picked out an unmarked Danny Welbeck in added time and the substitute turned to lift his effort past Lukasz Fabianski.
Arsenal were caught out on the counter for the opener, West Ham captain Marko Arnautovic slotting low past Petr Cech after exchanging passes with the impressive Felipe Anderson, who was allowed to carry the ball from deep.
Yet having both full-backs pushed up led to the equaliser five minutes later, Nacho Monreal firing into an unguarded net after Bellerin's cross deflected into his path off Alex Iwobi.
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"It was very important to win, and we showed that we can win in difficulty, after 1-0 in the first half," Emery told BBC Sport.
"We suffered with the transition in the first half, but we improved in the second half and controlled it more like we want.
"We need to improve in this transition to get the balance."
Arnautovic and Robert Snodgrass should have put West Ham in front again but shot straight at Cech, with former Gunner Lucas Perez also slicing wide late on in search of an equaliser as the visitors fell to a third straight defeat under their new manager Manuel Pellegrini.
The Hammers drop to bottom of the Premier League, while Emery's first win since replacing Arsene Wenger in May moves Arsenal up to 11th.
Frail Arsenal need defensive improvement
The first half at times resembled a contest to determine which side's defence were more inept, with Arsenal edging it.
Their tactic of playing out from the back with Shkodran Mustafi and Sokratis Papastathopoulos split either side of Cech was too easy for West Ham to disrupt, particularly with neither centre-back looking mobile enough.
It led to several chances for the visitors, as did having full-backs Monreal and Bellerin pushed high - the latter especially offered plenty of threat going forward but it meant West Ham had plenty of space to counter-attack into.
Anderson exploited that space brilliantly throughout, driving at the Arsenal defence before playing a one-two with Arnautovic through Mustafi's legs and allowing the striker to pick his spot in the bottom corner from outside the area.
Elsewhere, Arsenal failed to pick up runners, with the offside Arnautovic wisely leaving Michail Antonio's through ball for the overlapping Snodgrass, only for the Scot to pick out Cech.
The hosts escaped with a win here but better sides will both carve them apart and finish their chances if the defending does not improve.
Or will they just try to outscore opponents?
Arsenal showed glimpses of their attacking potential and started the game by dominating possession and winning tackles in midfield, with 19-year-old Frenchman Matteo Guendouzi impressing again.
Yet that spell in control lasted only 10 minutes and it took Diop's bizarre own goal to spark the Gunners' attack once more, with Lacazette and Aubameyang both going close as the Hammers reeled.
Aaron Ramsey had a goal rightly disallowed for offside and saw a low drive superbly saved by Fabianski late on as the midfielder proved an adequate replacement for Mesut Ozil, who Emery said was left out due to illness.
Henrikh Mkhitaryan produced a decent display, while summer signing Lucas Torreira was a positive influence after replacing the tiring Guendouzi, suggesting the makings of a fine midfield if Emery can find the right combination.
Mixed Hammers fall to the bottom
While Arsenal's defence was more obviously shambolic in the first half, it was West Ham's that ultimately determined the result.
In the build-up to his own goal, Premier League debutant Diop miscued a clearance straight up in the air, allowing Rasmey to head the ball to Lacazette, who turned and found the net via the hapless retreating defender.
Elsewhere, the visitors could could have conceded more but for a fine performance by Fabianski against his former side.
The same could not be said for Jack Wilshere, the former Gunner kept largely anonymous throughout and cutting a frustrated figure as he picked up a late booking.
Missed chances aside, West Ham showed promise going forward, with Anderson offering his side pace and crisp passing, while Arnautovic was a threat until he was forced off with a knee injury in the second half.
They need that knock to be nothing too serious to avoid a difficult start to the season turning into a real struggle.
Man of the match - Felipe Anderson (West Ham)
'It will come soon' - match reaction
Arsenal manager Unai Emery, speaking to BBC Sport: "We are happy because it is the first three points after two defeats.
"In the second half we wanted to find more balance on the pitch and we did. It was a difficult match, for that we are happy but there are things to improve."
West Ham boss Manuel Pellegrini, speaking to BBC Sport: "They won 3-1, but if you analyse the game you see we had so many options to score the goal.
"I am very happy in the way we played, we played well with good pace and movement, so I am sure soon it will come.
"The team played well, it was a team that defended well. I am happy as a team we are improving on a lot of things we are trying to do."
Gunners extend London derby run - match stats
- Arsenal are unbeaten in their last 10 Premier League London derbies at the Emirates, winning seven and drawing three.
- West Ham United have lost their first three games of the Premier League season for the third time this decade (also 2010-11 & 2017-18) after doing this just five times between 1920 and 2010.
- Marko Arnautovic has been directly involved in 19 Premier League goals since the start of last season (13 goals, 6 assists), eight more than any other West Ham player.
- Issa Diop is only the eighth player to score an own goal on his Premier League debut and the first to do so for West Ham United in the competition.
- Arsenal striker Danny Welbeck netted his third Premier League goal against West Ham United; only against Aston Villa (4) and Everton (4) has he scored more.
Arsenal are at Cardiff on Sunday, 2 September (13:30 BST), while West Ham are on EFL Cup duty against AFC Wimbledon on Tuesday (19:45) before hosting Wolves in the league on Saturday, 1 September at (15:00).
- 5PapastathopoulosBooked at 90mins
- 29GuendouziSubstituted forTorreiraat 56'minutes
- 17IwobiSubstituted forLacazetteat 45'minutes
- 14AubameyangSubstituted forWelbeckat 75'minutes
- 24FredericksBooked at 80mins
- 23DiopBooked at 41mins
- 11SnodgrassSubstituted forYarmolenkoat 75'minutes
- 15C Sánchez
- 19WilshereBooked at 88mins
- 30AntonioSubstituted forPérezat 63'minutes
- 8Felipe Anderson
- 7ArnautovicSubstituted forHernándezat 59'minutes
- Graham Scott
- Shots on Target
Second Half ends, Arsenal 3, West Ham United 1.
Attempt blocked. Carlos Sánchez (West Ham United) right footed shot from outside the box is blocked.
Foul by Granit Xhaka (Arsenal).
Andriy Yarmolenko (West Ham United) wins a free kick on the right wing.
Goal! Arsenal 3, West Ham United 1. Danny Welbeck (Arsenal) left footed shot from the centre of the box to the bottom right corner. Assisted by Héctor Bellerín with a cross.
Foul by Aaron Ramsey (Arsenal).
Issa Diop (West Ham United) wins a free kick in the defensive half.
Foul by Henrikh Mkhitaryan (Arsenal).
Arthur Masuaku (West Ham United) wins a free kick in the defensive half.
Attempt missed. Chicharito (West Ham United) header from the centre of the box misses to the left. Assisted by Ryan Fredericks with a cross.
Sokratis (Arsenal) is shown the yellow card.
Foul by Sokratis (Arsenal).
Chicharito (West Ham United) wins a free kick in the defensive half.
Jack Wilshere (West Ham United) is shown the yellow card for a bad foul.
Danny Welbeck (Arsenal) wins a free kick in the defensive half.
Foul by Jack Wilshere (West Ham United).
Delay over. They are ready to continue.
Delay in match Granit Xhaka (Arsenal) because of an injury.
Corner, Arsenal. Conceded by Lukasz Fabianski.