Fiorentina captain Cristiano Biraghi was left bleeding after being hit by a missile thrown by West Ham supporters.
The left back was taking a corner just before the 35 minute mark when a number of objects reigned down on him from the stands. And blood then visibly streamed from his face as referee Carlos del Cerro halted the action and threatened to take both sides off the field.
Play resumed after lengthy consultation between the officials and team captains, and the subsequent stadium announcement was made: "An important security announcement. Stop throwing objects onto the pitch. Please respect players and the officials."
Prior to the resumption, Declan Rice led a host of Hammers players in appealing for calm from fans. Biraghi, who initially responded to being hit by sarcastically clapping the London club's supporters, was deemed ok to continue after treatment.
A frenzied few minutes followed with both sides seemingly unsettled into the incident, before the Premier League side were fortunate to go in 0-0 at the break. Luka Jovic pounced before the break to head in from close range, but his goal was ruled out for offside.
During the interval, BT Sport pundit Joe Cole lamented the trouble: "It's moronic behaviour, it really is," he said. "We've praised the West Ham fans so far and rightly so, they've been very well behaved. But it only takes a handful of idiots to do something like that.
"These lads, coming onto the pitch to play football, I've had it myself when you get hit with things, if someone takes someone's eye out....it needs to be sorted out and taken out the game."
Earlier in the day, Fiorentina fans has been criticised for attacking their rivals with chairs, as well as releasing flares and fireworks. Police later confirmed 30 men had been apprehended in the aftermath of the attack.
It's the second consecutive European tie where the action on the field for West Ham as been overshadowed by incidents in the stands. Their semi final second leg win at AZ Alkmaar was marred by the away faithful being attacked after the final whistle by a group of 'ultras' from the Dutch club.
