By Thomas Grove and Gabriela Baczynska
MOSCOW (Reuters) - Tens of thousands of demonstrators marched though Moscow under streaming banners, flags and balloons on Saturday to demand an end to President Vladimir Putin's long rule and show their protest movement remains strong.
Protesters chanted "Russia without Putin!" in the first big rally since June, defying increasingly tough Kremlin measures against the opposition since the former KGB spy began another six years in the Kremlin in May.
Some demonstrators released colorful balloons decorated with ski masks like the ones worn by the Pussy Riot punk band, three of whose members have been jailed for singing a profanity-laced protest song against Putin in a church.
Witnesses said opposition leaders appeared to have achieved their goal of attracting at least 50,000 people, enough to maintain the momentum of their nine-month-old movement but almost certainly too few to increase alarm in the Kremlin.
"The summer has gone, three months since our last march. Not a single demand has been met ... on the contrary, repressions have only gathered pace, more people have been arrested," far-left leader Sergei Udaltsov told a rally after the march.
Recalling a stunt in which Putin flew in a light aircraft alongside migrating cranes this month, Udaltsov said: "The president has detached himself from reality. He flies with cranes and just spits on the people from above."
Organizers also released white balloons and doves into the cloudy sky before opposition leaders led the march down a leafy central Moscow boulevard behind a long banner declaring: "For early elections! Against repression!"
Some wore T-shirts demanding the release of 17 protesters facing trial over a rally on May 6 that ended in clashes with police. Others said it was vital to keep attending protests to inspire others to join them and to defy the Kremlin's pressure.
"We will come here as if to work. It must be part of our everyday life," said Alexei Navalny, an opposition blogger and protest organizer. "I want each of us to ask ourselves when we look in the mirror in the morning what we can do for freedom."
Riot police were out in force but stood by watching for most of the day, until witnesses said they detained Udaltsov and several others around 10 p.m. (1800 GMT), the time by which city authorities had said the eight-hour protest must end.
The protesters say Putin's return to the Kremlin after four years as premier is a setback for democracy. He could now extend his rule of Russia to 24 years if he wins another term when his mandate expires in 2018.
This would mean ruling longer than Soviet leader Leonid Brezhnev, and opponents fear political and economic stagnation.
"People who lived in the Soviet Union are tired of absolute rulers. We're tired of a police state," said Alexander Kokhmansky, 79, as he marched.
Sergei Yevseyev, 35, said he was protesting against "the total lawlessness, total corruption, the lack of civil freedoms, the absence of independent courts and social injustice."
There was also more focus on social problems than at previous protests, and more communists marched this time with nationalists, liberals, gay rights campaigners and leftists.
Other marchers wanted to show solidarity, without being confident the opposition can achieve much after failing to select a leader or touch off protests outside the big cities.
The demonstrations began last December over allegations of fraud in a parliamentary election won by Putin's party and turned into the biggest protests against him since he first became president in 2000, at times drawing up to 100,000 people.
Putin, who is 60 next month, dismisses the protesters as a minority who do not have wide support across Russia. Apart from some minor electoral reforms at the peak of the protests last winter, the Kremlin has resisted calls for political change.
Putin's spokesman, Dmitry Peskov, told Interfax news agency as saying the president had been working in his residence in the resort of Sochi and paid no attention to the rally.
"Alas we are unable to monitor that action," he said.
CONCERN OVER CLAMPDOWN
Even so, opinion polls show Putin's ratings, although still high by Western standards, are falling.
Speakers at the rally criticized Putin over new laws increasing protesters' fines, stiff punishment for defamation and new controls on foreign-funded campaign groups.
Opposition deputy Gennady Gudkov was expelled from parliament on Friday on allegations of continuing business activities while holding a seat in the house and Navalny could face 10 years in jail on theft charges. Both deny the charges.
"There is no more constitution in Russia. There are no more rights and there is no more parliament worthy of respect," Gudkov told the crowd.
About 2,000 people protested in St Petersburg, witnesses said, and Udaltsov said police detained some 15 protesters in the central city of Nizhny Novgorod. A small protest took place in Yekaterinburg in the Ural Mountains, and witnesses said a there was a small rally in the far east city of Vladivostok.
(Additional reporting by Maria Tsvetkova, Nastassia Astrasheuskaya and alissa de Carbonnel; Writing by Timothy Heritage, Editing by Steve Gutterman)