MOSCOW (AFP) - Zenit St Petersburg supporters invaded the pitch on Sunday and attacked Dynamo Moscow's captain as their side were headed for a shock home defeat in a crunch title game.
Andre Villas-Boas's side were losing 4-2 in the 87th minute when St Petersburg fans launched their assault and forced the game to be abandoned.
One fan punched Dynamo skipper Vladimir Granat to the head twice. Granat was helped from the field in a daze. Reports said he suffered concussion.
Dynamo officials said they would launch legal action.
"The attack on Granat is without doubts a criminal offence," the Interfax news agency quoted one Dynamo chief as saying.
"I believe that the law-enforcement authorities should investigate this case."
Zenit said the club was trying to identify the fan who attacked Granat.
"The fans unruly behavoir is serious blow for the club's reputation," Zenit press service chief Yevgeny Gusev said.
"The club and the police are trying to identify the person who attacked Granat. We're also preparing evidence for launching legal action againt him.
"We believe that the persons guilty of this incident will suffer punishment for their actions."
Zenit now face an automatic 3-0 defeat and other sanctions.
The head of the Russia's Football Union (RFU) disciplinary panel Artur Grigoryants said a decision on the case would be taken on Wednesday.
"We need to collect the documents and to hear the match officials' evidence," Grigoryants was quoted as saying by ITAR-TASS news agency.
Last season, Zenit were handed a 3-0 technical defeat at Dynamo after a firecracker thrown by Zenit fan injured Dynamo goalkeeper Anton Shunin.
Zenit were also sentenced to a 1.3-million-rouble (S$45,970) fine and played two Premier League matches behind closed doors. Zenit finished second, two points behind champions CSKA Moscow.
This year's title could again be decided by the punishment.
Villas Boas' side had been undefeated in seven games going into the match and went one up after six minutes. Portuguese midfielder Miguel Danny fired in the rebound after Dynamo goalkeeper Vladimir Gabulov deflected Salomon Rondon's close-range shot.
Dynamo pulled level in the 35th minute. Artur Yusupov fired the ball in from just inside the penalty area with a help of a ricochet off Zenit Italian wingback Domenico Criscito.
In the 57th minute Argentinian back Leandro Fernandez put Dynamo ahead with a 12-metre header from a corner. Ecuadorian midfielder Christian Noboa made it 3-1 for Dynamo with another header eight minutes later.
German forward Kevin Kuranyi scored Dynamo's fourth in the 71st minute while Russian internationanl Alexander Kerzhakov pulled one back for Zenit in the 84th minute.