* Opposition marks anniversary of rally that turned violent
* Protests have faded since peak early last year
* Putin denies repression, has tightened grip on power (Adds quotes from protest leader Navalny)
Thousands of Russians demanded an end to President Vladimir Putin's long rule and said they would not let him "turn the country into another Gulag" at a rally on Monday intended to revive a flagging protest movement.
But many Russians are frustrated by the opposition's failure to turn big rallies last year into a sustained challenge to Putin, and the joyous mood of the initial protests has given way to a subdued realisation that his grip on power has tightened.
Anti-corruption blogger and protest leader Alexei Navalny led chants of "Putin is a thief" and "Russia will be free".
"We will drive him out, throw him from the Kremlin. We don't want him to command us," Navalny said.
Riot police lined up in view of the Kremlin while a helicopter buzzed above the crowd that was bigger than expected, but markedly less enthusiastic than when the protest movement started 18 months ago, sparked by allegations of vote fraud.
One protester dressed as the Grim Reaper and held a scythe. Others waved banners declaring "Freedom, elections, democracy, peace" on the same square where baton-wielding police broke up a protest a year ago on the eve of Putin's inauguration.
"A year has passed and nothing has changed. The protests have diminished and the repression continues - they are jailing people," said Roman Bryzgalov, 24. "I have friends who were jailed for the protest here a year ago - it's personal for me."
His defiance was typical of hardcore protesters who are determined to keep pressing for change even though Putin, now 60, has responded to none of their main demands for change and shows no sign of doing so in his third term as president.
Putin's critics saw the use of force against protesters last year as a shift towards intimidation and repression and a turning point in the Kremlin's tactics.
"FIGHTING FOR RUSSIA"
State-run RIA news agency said police detained six people for disorder.
Two people have been jailed over last year's rally and 28 are awaiting trial, while several opposition leaders, including Navalny, face criminal charges which they say are politically motivated and could see them jailed for years.
"We are fighting for Russia, for our country, our future," Gennady Gudkov, a protest leader and former member of parliament, told the crowd - estimated by police at 7,000 and by reporters at up to about 15,000.
"We won't let them turn the country into another Gulag," he said, reiterating comparisons between Putin's methods and those of Soviet dictator Josef Stalin, who sent millions of opponents to their deaths in the Gulag network of labour camps.
Putin, who in 13 years of power has succeeded in sidelining his opponents, has at times mocked the opposition, and at others ignored them as he set about boosting support among provincial blue-colour workers who are his main power base.
He has denied that the wave of court cases against opposition leaders are politically motivated.
For every protester who turned up on a chilly spring evening in central Moscow, there were many more who stayed at home after attending the initial protests that began against Putin after a parliamentary election marred by fraud.
They have given up attending rallies because they hold out little hope of change after a year in which Putin has reasserted his authority, and their anger has reverted to apathy. Some are also deterred by the fear of violence and persecution.
Political satirist Viktor Shenderovich urged demonstrators not to give up but acknowledged that the high hopes of many protesters at the initial rallies had not been fulfilled.
"It's hard not to fear the OMON (riot police) and trials - the whole state machine they are throwing at us," he said.
In other protests, a banner demanding "Freedom for the May 6 prisoners!" was unfurled from the top of an apartment block on one of Moscow's main streets and activists plastered the names of protesters awaiting trial over street signs in Yekaterinburg.