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, believed to be a Liverpool fan, suffered a head injury and is being treated in hospital, 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.
Det Insp Paul Speight of Merseyside Police said: "Witnesses report that the victim was hit with a belt and then fell to the ground.
"His condition is currently described as critical and his next of kin have been informed."
There were clashes between Liverpool and Roma supporters shortly before kick-off.
About 80 Roma fans are thought to have used a side road to reach an area where Liverpool supporters had gathered.
One supporter was seen motionless on the road and BBC Sport footage showed another fan carrying a hammer.
Seven men, aged between 20 and 43, have been arrested for offences including affray, assault, possession of offensive weapons and criminal damage.
Ch Supt Dave Charnock said fans from both sides had been "involved in isolated pockets of disorder".
He said detectives were also investigating the use of flares outside Anfield stadium prior to the game.
"A number of those in the crowd that had gathered to greet the Liverpool team coach were in possession of flares and work is ongoing to identify those responsible and to establish if offences have been committed".
Trouble broke out behind the Kop End, with reports of Roma supporters attacking home fans.
BBC Sport news correspondent David Ornstein 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.
Sunday Times football correspondent Jonathan Northcroft 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.
There was an increased police presence around the stadium after Manchester City's team bus was attacked before their Champions League quarter-final with Liverpool earlier this month.
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.