UPDATE 1-Russian police block new anti-Putin rally
* Moscow protest plans fizzle in face of police presence
* Dozens detained in St Petersburg
* Putin files candidacy papers in presidential bid
* Russia lashes out at U.S. criticism (Adds demonstrations, police show of force, Gorbachev)
By Guy Faulconbridge and Steve Gutterman
MOSCOW, Dec 7 (Reuters) - Plans for big new protests against Vladimir Putin fizzled on Wednesday after a show of force by Russian police who have detained more than 1,000 people in a crackdown since a parliamentary election dismissed by Kremlin foes as a fraud.
It was a setback for government opponents seeking to channel public anger over the election, widely seen as slanted in favour of Prime Minister Putin's ruling United Russia party, into a powerful protest movement.
Putin pressed ahead with his bid to return to the presidency next year, filing papers to register his candidacy, while former Soviet leader Mikhail Gorbachev suggested the official results of Sunday's vote were a "lie" and called for a new election.
A day after police dispersed protesters in central Moscow and seized others before they could even reach the rally, detaining more than 300, opposition activists had planned a new demonstration at the same site 24 hours later.
But hundreds of helmeted riot police blocked off the square after nightfall, pushing back reporters and shouting through loudspeakers: "Respected citizens, please do not stop, walk on your way so as not to hinder others."
Three youths emerged near a subway station entrance, chanting: "We want free elections!". Riot police marched them off to one of the dozens of police buses and truck that lined the streets nearby.
In St Petersburg, about 250 people protested, most of them youngsters, shouting "Shame!" Police detained about 70.
Kremlin opponents are trying to maintain momentum after 5,000 people turned out on Monday night for the largest opposition protest in Moscow in years, demanding fair elections and chanting "Russia without Putin!".
Police and Putin's spokesman have said unapproved protests will be stopped. The Interior Ministry said some 50,000 officers and 2,000 ministry troops remained in Moscow after the election.
A test of the drive to pressure Putin with street protests will come on Saturday, when opponents hope for a big turnout at a rally near the Kremlin.
Two protest leaders arrested after Monday's rally will still be in jail then. A judge on Wednesday rejected appeals filed by Alexei Navalny and Ilya Yashin against the 15-day jail terms they received the previous day.
Monday's protest, fanned by fraud accusations that spread on the Internet, underscored anger at United Russia and unhappiness among some Russians at the prospect of Putin's almost certain return to the Kremlin in a March presidential vote.
Voters bruised Putin in Sunday's election by sharply reducing his party's majority in the State Duma lower house.
Undeterred, Putin filed candidacy papers for the March 4 presidential vote, submitting the documents in a brief, nearly silent visit to the Central Election Commission headquarters.
Registration as a candidate is a formal step towards what could be another 12 years in the top job for Putin, 59, who was president from 2000 to 2008 and is now prime minister but remains Russia's paramount leader.
Putin and President Dmitry Medvedev, the protege he tapped as successor when the constitution barred him from a third successive term in 2008, said in September they plan to swap jobs next year, with Medvedev taking over as prime minister.
Putin remains Russia's most popular politician and is likely to win a six-year presidential term, after which he could run again, potentially serving until 2024.
But the sharp decline in support for his ruling party was a sign of frustration with the political system he has put in place, in which many Russians feel they have no influence.
United Russia received 49.4 percent of the votes in Sunday's election and will have 238 seats in the 450-member Duma, down from 315 now. Rival parties that won seats and the marginalised politicians leading street protests say even that result was inflated by fraud such as ballot-box stuffing.
GORBACHEV WANTS NEW ELECTIONS
"With each day, more Russians do not believe that the declared results are honest," Gorbachev was quoted as saying by Interfax news agency, adding that ignoring public opinion discredited the authorities and destabilised the situation.
"And so I think it is necessary to annul the results of this vote and announce new elections," he said.
Grigory Yavlinsky, whose liberal Yabloko party fell short of the 5 percent of votes needed to win Duma seats, said those who won seats should force a new election by refusing to take them.
"If you think that vote-rigging was substantial, then get out of the State Duma ... then we can have new elections that will pass off better," Yavlinsky told reporters.
The Communist Party will have 92 seats, the left-leaning Just Russia will have 64 and the nationalist LDPR 56. Kremlin critics say all three parties are part of a system managed by the Kremlin and present no real threat to Putin's rule.
The election and the police crackdown has increased tension between Russia and the West, already wary about Putin's planned return to the Kremlin.
International observers said the campaign was slanted in favour of United Russia. The United States and the European Union voiced concern about the conduct of the election and the treatment of protesters. Russia called the U.S. criticism "unacceptable". (Additional reporting by Thomas Grove, Darya Korsunskaya and Jennifer Rankin, writing by Steve Gutterman, editing by Timothy Heritage)