The Russian opposition Tuesday defiantly vowed to wage a sustained campaign of protests after police detained hundreds in rallies against Vladimir Putin's crushing victory in presidential polls.
Protest leaders were holding talks with the Moscow authorities to agree a venue for a mass protest on Saturday to keep pressure on Putin ahead of his return to the Kremlin for at least six more years.
The tense demonstration on the night after Sunday's election descended into chaos and violence when police roughly arrested a hard core who had tried to stage a sit-in protest on a central Moscow square.
Police said they arrested 620 people there and at a similar event in Saint Petersburg. Those detained in Moscow included the anti-corruption crusader Alexei Navalny -- the movement's charismatic but divisive figurehead.
All were released by Tuesday, police said, but several face court hearings that could hand them 15-day jail terms.
"Tens of thousands will be coming out on the streets of Moscow and other cities and refusing to leave," Navalny told reporters after spending most of Monday night in detention. "We will keep doing this until our demands are met."
Moscow police said they detained around 20 people Tuesday evening on the same square as Monday's protests, adding that they had learned of a planned "provocation" from social networking websites.
The spectre of unrest has overshadowed what was meant to be Putin's triumphant return to a post he held with virtually no dissent from 2000 to 2008 before his four-year stint as prime minister.
The ex-KGB spy is due to be sworn in at the Kremlin on May 7.
The last Soviet leader Mikhail Gorbachev on Tuesday slammed Putin for dismissing the opposition movement as "enemies" in speeches at rallies during his campaign.
"I was amazed when he said that the first time, calling them enemies," he told Moscow Echo radio station.
He said Putin's comments were "humiliating" and Russia's rulers "must apologise."
The opposition has raised the alarm over the police force used at the rallies for the first time since they grew into the first sustained post-Soviet protest movement three months ago.
US Ambassador Michael McFaul also called the scenes "troubling" on his Twitter account.
But Putin's spokesman Dmitry Peskov told ITAR-TASS that police displayed the "highest professionalism" and behaved in a "legitimate and effective manner" at the Moscow event.
Putin won Sunday's election with 63.6 percent of the vote and will serve for a six-year term that if he is re-elected could extend to 2024 -- making him Moscow's longest-serving ruler since Stalin.
But European monitors have raised concerns and the opposition, whose leaders were excluded from the polls, have vowed to make protests a permanent feature of the Russian strongman's new presidency.
The next big event in Moscow is set for Saturday when organisers hope to lead tens of thousands on a march along the central Novy Arbat street. The city has offered a more remote location and no final venue has yet been agreed.
"The awakening of society," the Vedomosti daily said in an editorial about the difficulties Putin faces on his Kremlin comeback.
"The possible return of Putin for two more terms has brought on fears of stagnation and despair," it said.
But some have expressed fears that the rallies may be losing the ironic edge that kept them popular with crowds of all ages and could turn more urgent and volatile as the reality of Putin's new term sets in.
Monday's event in Moscow featured the first speaking appearance by tycoon and third-place election candidate Mikhail Prokhorov -- an independent who has vowed to build his own party after winning almost 8.0 percent of the vote.
Some analysts believe the Kremlin privately backs Prokhorov in the hope that he can win over some of the movement's supporters and thus calm the tensions on the streets.
The tycoon has repeatedly called himself an "alternative" to Putin and does not view himself as an outright member of the opposition.
He defended democratic freedoms at the rally and later urged the police to exercise more restraint.
"I am certain that the use of force and detention of opposition politicians could have been avoided," Prokhorov wrote on his Twitter account.