Ex-PM Mikhail Kasyanov has failed to win the leadership of Russia's oldest liberal party and has accused the authorities of splitting it.
Mr Kasyanov (right) is considering whether to launch a new party
Mr Kasyanov - a fierce critic of President Vladimir Putin - had hoped to be elected as leader of the Democratic Party (DPR) at a congress in Moscow.
But the gathering was disrupted, and some party members held a separate meeting and elected a rival leader.
Mr Kasyanov hopes to unite liberal forces and run for the 2008 presidency.
The former prime minister had been widely expected to win the DPR leadership on Saturday, which would have strengthened his presidential bid.
But a large group of delegates locked themselves inside a downtown conference hall, which had been chosen as a venue for the party congress weeks ago.
"The hall was taken over during the night by people... and they placed guards and are not allowing anyone in," Mr Kasyanov's spokeswoman Tatiana Razbash was quoted as saying by the Associated Press news agency.
When Mr Kasyanov, 47, and some of his supporters moved to another site in the capital, they did not have enough party members present to allow a vote on the leadership to go ahead.
Meanwhile, a vote was held at the rival event, and 35-year-old Andrei Bogdanov was elected as DPR's leader.
A number of party delegates alleged that they had been offered bribes of up to $10,000 each to withdraw their support for the former prime minister.
"The authorities defeated us," Mr Kasyanov said later, adding that he would consider whether to contest Mr Bogdanov's election, or launch a new party.
Mr Kasyanov is hoping to form a broad coalition of opposition parties to challenge the Kremlin in 2008, when President Putin is set to step down after serving two terms.
Mr Kasyanov was President Putin's prime minister for four years until his sacking last year, for which no reason was given.
Now Mr Kasyanov says his former boss is curtailing democratic freedoms in Russia.
Since he began voicing criticism of the Kremlin, Mr Kasyanov has found himself under increasing pressure.
There is talk of a possible criminal case against him over alleged corruption.
Mr Kasyanov cannot get airtime on Russia's main national TV networks, the BBC's Steve Rosenberg says.