Volgograd's ousted mayor was purged from the ruling United Russia party late last week, as local residents staged rallies in support of and against him.
Roman Grebennikov, fired by regional Governor Anatoly Brovko on Tuesday, was expelled from United Russia on Thursday at an extraordinary meeting of its Volgograd branch, which cited "discrediting the party" as the reason, Interfax reported.
Dmitry Lunev, a Volgograd lawmaker with United Russia, was expelled from the party Friday for supporting Grebennikov, Interfax said.
Deputy Mayor Sergei Sokolov became acting mayor Friday, the city administration said in a statement carried by its official mouthpiece, Volgadmin.ru.
Sokolov, a former Gazprom employee who RIA-Novosti said was initially reluctant to become the acting mayor for unspecified reasons, will hold the job until mayoral elections, the date for which remains unannounced.
His former boss refused to go down without a fight, filing a challenge over his removal in a Volgograd district court on Thursday, Interfax said, without elaborating whether a date had been set for a hearing.
Grebennikov said earlier that his removal was ordered when he was on sick leave, which is illegal, and the Volgograd legislature was not consulted about the move, as city legislation requires.
Grebennikov's supporters started a series of daily rallies, set to last until March 8. Friday's event attracted some 300 protesters, Interfax said.
About the same number of people gathered elsewhere in the city on Friday to support the mayor's ouster, citing his weak leadership, the report said. The anti-Grebennikov rally, unlike the pro-mayor event, was not sanctioned by authorities. Police did not disperse either rally.
The ousted mayor has but a slim chance of returning to the job, said regional analyst Alexander Kynev.
"I don't know any cases when a fired mayor has been able to regain his post in court," Kynev said by telephone.
A Kremlin-appointed governor dismissing an elected mayor "is an alarming signal on the eve of elections," Kynev said, adding that Volgograd, a city of 1 million, may become another locality where the mayor is chosen by the local legislature instead of the public and shares his duties with an unelected city manager.
Local opposition complained that the conflict shows how local officials ignore the populace.
"Our opinion is not valuable for the governor since his only voter is the president," Galina Boldyreva, a head of the Volgograd regional branch of the Yabloko party, said by phone.
Boldyreva noted that Grebennikov, who was elected in 2007 on the Communist ticket, had been acting more independently before his ouster. "He invited us and other opposition parties to work together," she said.