Italian football fans have reacted violently inside and outside stadiums following the police shooting of a Lazio supporter.
Gabriele Sandri, 26, was shot in what police called a "tragic error" as they tried to stop violence between rival fans at a motorway stop in Tuscany.
A match between Atalanta and AC Milan was stopped as fans and police clashed. There was violence at other games.
Later hundreds of fans rampaged in Rome and there were more protests in Milan.
Police chiefs, politicians and football administrators will be meeting on Monday to seek to limit the damage from the weekend's incidents.
Bus torched
The worst violence was in the capital, where hundreds of armed fans attacked a police barracks and the Italian Olympic Committee headquarters.
In pictures: killing aftermath
Sunday's late match between AS Roma and Cagliari had been postponed as a precaution but fans wielding rocks and clubs turned up outside the Stadio Olimpico.
Security guards in the Olympic headquarters barricaded themselves in as fans outside smashed windows and burned vehicles as they clashed with police.
The mob blocked off one end of a bridge over the Tiber and ordered motorists to leave the area.
A bus was torched and several people including police were injured.
There were also angry scenes in central Milan near the offices of the broadcaster RAI, as fans hurled rocks at a police station and beat up two journalists.
In Bergamo, where Atalanta were playing AC Milan, police and fans clashed ahead of the match.
The game was abandoned 10 minutes after kick-off, when fans tried to smash down a barrier and force their way onto the pitch.
In Siena, supporters shouted "murderers" at police.
There was also violence at lower league games in southern Italy.
Seven of the top league games started 10 minutes late with players wearing black armbands although atmospheres remained tense.
Italian Prime Minister Romano Prodi called for a full investigation into the shooting and said the violence was "very worrying".
'Tragic error'
Mr Sandri, a 26-year-old disc jockey from Rome, was a Lazio fan on his way to a match with Inter Milan.
Lazio fans and supporters of Juventus on their way to a match at Parma reportedly clashed at the service station near Arezzo.
Mr Sandri was apparently shot while in a car outside the motorway restaurant.
Police suggested he may have been killed by a warning shot.
The exact details of the shooting are unclear and an investigation is under way.
"It was a tragic error," said Arezzo police chief Vincenzo Giacobbe.
"Our agent had intervened to prevent the brawl between these two groups, who had not been identified as fans," Mr Giacobbe said, according to the Italian news agency Ansa.
The Inter-Lazio game was postponed.
In April the Italian government introduced a law aimed at stamping out football hooliganism.
It was enacted after a policeman was killed in rioting at a match in Sicily in February.
The BBC's Frances Kennedy in Rome says that despite the new anti-hooliganism measures, Sunday's explosion of anger shows that violence is never far from the surface.
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