Arsenal recorded their first Premier League win under new manager Unai Emery after Issa Diop's own goal and Danny Welbeck's late strike saw West Ham fall to their third straight defeat.

The defensively frail Gunners came from behind to lead in fortuitous fashion as Alexandre Lacazette's volley across the area deflected in off Diop.

Hector Bellerin then picked out an unmarked Welbeck in added time and the substitute turned to lift his effort past Lukasz Fabianski.

West Ham captain Marko Arnautovic put the visitors in front on 25 minutes, exchanging passes with the impressive Felipe Anderson before slotting low past Petr Cech.

Nacho Monreal equalised five minutes later, firing into an unguarded net after Bellerin's cross deflected into the defender's path off Alex Iwobi.

Both Arnautovic and Robert Snodgrass then shot straight at Cech as the Hammers missed several chances to take the lead again, with former Gunner Lucas Perez also slicing wide late on in search of an equaliser.

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 overly comfortable or mobile enough on the ball.

It led to several chances for the visitors, as did having full-backs Monreal and Bellerin pushed up 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, carrying the ball from deep to drive 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, reportedly 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 bring it together.

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 and sent the visitors bottom of the table.

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)

'Very important to win' - match reaction

Arsenal manager Unai Emery, speaking to BBC Sport: "We are happy because it is the first three points after two defeats. It was very important to win, and we showed in the match that in the difficulty we can win, after 1-0 in the first half.

"We suffered with the transition in the first half, but we improved in the second half and controlled more like we want. In the second half we wanted to find more balance on the pitch and we did.

"We need to improve. To improve in this transition to get the balance, to get more control with the ball but also, West Ham are a good team, 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.

What's next?

Arsenal are at Cardiff on Sunday, 2 September at 13:30 BST, while West Ham are on EFL Cup duty against AFC Wimbledon on Tuesday (kick-off 19:45 BST) before hosting Wolves in the league on Saturday, 1 September at 15:00 BST.