Arsenal went on to win the tie in a penalty shoot-out
An Arsenal football fan suffered a suspected stab wound when the minibus he was on was ambushed ahead of the club's Champions League tie in Rome.
The bus was attacked by Roma fans after the driver became lost near the Olympic Stadium, the British Embassy said.
The victim was treated inside the stadium for a cut to his leg.
Prior to the match, English fans had been advised to avoid areas where Roma's hooligan following, the Ultras, are known to congregate.
An Arsenal spokeswoman said: "A male Arsenal fan was believed to be stabbed outside the stadium.
"Roma fans threw stones at a bus, smashing windows, and an individual allegedly entered the bus and stabbed a fan."
A British Embassy spokesman said: "This bus got a bit lost around the stadium and some Roma fans tried to get on the bus.
"One managed to get on to the bus and there was a fight.
"The driver managed to get away when the Roma fan was pushed out of the bus."
He added: "We don't know if (the injury) was a stabbing or glass from a smashed window."
Roma won the match 1-0, cancelling out Arsenal's 1-0 win in the first leg, but the London club won a penalty shoot-out 7-6 to book a place in the quarter finals.