A 53-year-old man is in a critical condition after he was attacked outside Anfield before Liverpool's Champions League semi-final match with Roma.
Two men, who are aged 25 and 26, and come from Rome, have been arrested on suspicion of attempted murder.
The victim suffered a head injury and is being treated in hospital, Merseyside Police said. In a statement Liverpool said it was "shocked and appalled" and would be offering the victim "our full support". The attack happened outside the Albert pub in Walton Breck Road at about 19:35 BST.
Paul Speight of Merseyside Police said: "Witnesses report that the victim was hit with a belt and then fell to the ground.
One supporter was seen motionless on the road and a footage showed another fan carrying a hammer.
Trouble broke out behind the Kop End, with reports of Roma supporters attacking home fans. A witness said the pre-match trouble started when a "group of Roma fans arrived via Venmore Street, burst on to Walton Breck Road and attacked, many using belts".
He said a Liverpool fan was knocked to the floor and received lengthy medical treatment. He added that there were "skirmishes, projectiles" and at least one person had "a hammer".
Our correspondent tweeted that the "attack was sudden", adding that the "Roma group were chased down the street before police arrived".
Police are appealing for anyone with video footage or information to contact them.
Liverpool were charged by European football's governing body Uefa for throwing cans, bottles and flares at the bus.
Roma supporters have been involved in violent attacks against English fans before - several Tottenham supporters were injured in Rome in 2012.