West Ham and Rapid Vienna fans clashed in ugly scenes inside the London Stadium during their Europa League tie on Thursday.
Police and stewards had to form a barrier between both sets of supporters in one corner of the ground as objects were thrown across the divide.
One fan could be seen in footage launching a large object into the melee while others chucked water bottles as officers and staff attempted to restore order.
The row had been sparked after West Ham's Declan Rice fired his side in front after 29 minutes.
Rapid Vienna fans reportedly stormed the home end to confront West Ham supporters before chaos ensued.
It is West Ham's first home fixture in Europe since 2016, and the club marked the occasion with a light show prior to kick-off.
Some objects are believed to have been thrown while the stadium floodlights were dimmed during the show.
Rice's goal then sent the home crowd into jubilation, getting them off to a superb start when he finished off Michail Antonio's cross.
But officers and stewards had to remain in position out of fear of trouble erupting again after missiles were exchanged.
Fortunately there were no more clashes during the match, though BT Sport reported there was some "unrest" at the final whistle.
The Hammers eventually ran out 2-0 winners as Said Benrahma scored a well-taken placed finish in the dying minutes to wrap up the victory and the three points as they maintained their perfect start in the competition.
It was a match they were expected to win comfortably with their opponents struggling badly in the Austrian league this season, and they almost went ahead after only eight minutes.
Rice's glancing header from Aaron Cresswell's free-kick beat Rapid goalkeeper Paul Gartler but came off the base of the far post.
The dominant Hammers were denied by the other post 15 minutes later, Craig Dawson thumping a header against it from a Cresswell corner. But Rice fired them ahead on the half-hour mark.
Michail Antonio chested the ball down from Andriy Yarmolenko's pass and raided down the left before crossing low for Rice to tap into an empty net.
It was the first home goal for a West Ham player in Europe, qualifiers aside, since Frank Lampard wrapped up a 3-0 UEFA Cup win over Croatian side Osijek in 1999.
West Ham's tempo dropped in the second half so Moyes made a triple substitution, sending on Jarrod Bowen, Tomas Soucek and Manuel Lanzini.
They had a let-off when German referee Tobias Stieler gave a penalty after Rapid sub Marco Grull went down under a non-challenge from Ben Johnson, with VAR correctly overturning the decision.
Bowen should have wrapped things up when he rounded Gartler only to fire over the crossbar. But Benrahma made sure it was goodnight Vienna when he cut inside and curled in the second in stoppage time.
There were more altercations in the Europa League in France as fans of Marseille and Galatasaray clashed before kick-off and during the game.
Flares were thrown between supporters before riot police had to be called.
At one stage the match was even temporarily suspended in the 37th minute until the situation had calmed after captains of both teams got involved and liaised with their own fans.
Elsewhere in the Europa League, Leicester City were beaten 1-0 away at Legia Warsaw following Mahir Emreli's first-half goal.
Rangers were also downed by the same scoreline against Sparta Prague as more unsavoury scenes marred the contest.
Celtic were also on the losing side as they suffered a heavy 4-0 defeat at the hands of Bayer Leverkusen.
