Analysts say some believe Sunday's election overstepped the mark
Russian opposition parties have walked out of parliament and threatened to raise mass demonstrations in protest at local elections they say were rigged.
Official results showed PM Vladimir Putin's United Russia party winning nearly every poll by a wide margin.
Some 135 out of 450 MPs walked out of parliament - the Duma - in a rare show of anger against the Kremlin.
"We will not sit in a room with fraudsters. We are leaving the room!" declared one opposition party leader.
Vladimir Zhirinovsky, head of the ultra-nationalist Liberal Democratic Party, demanded a recount at every polling station, after accusing United Russia of having "fraudulently appropriated" his party's votes.
The head of the Communist Party, Gennady Zyuganov, said his members would boycott the Duma until they were granted a meeting with President Dmitry Medvedev.
"Until we raise a mass protest, it will be impossible to correct the situation," he said.
In the vote for Moscow City Council, election officials said United Russia won 66% of the vote, taking all but three of the council's 35 seats.
"What's the point in having a legal system and laws if they only suit one party?" Vadim Solovyov, another Communist Party deputy, told Reuters news agency by telephone.
"Putin's system of government makes no sense and it simply doesn't work."
The third opposition party in the Duma, Just Russia, also joined the protest.
All three parties are normally reluctant to defy the Kremlin.
Other, liberal, opposition parties were obliterated at the local elections. They say it was by a combination of intimidation and fraud.
'Step too far'
Mr Putin dismissed the protests, saying: "Those who don't win are never happy."
He said he was "pleased" with the election results and advised those with complaints to go to the courts.
Analysts said even in a country that has long been suspected of choreographing elections, Sunday's vote seems to have been a step too far for some.
"The fact that there is a solidarity now between these parties is important political news; it means that the level of what is acceptable been surpassed," Moscow-based analyst Dmitry Oreshkin told the AFP news agency.