* Putin foes hold rally marking year of protests
* Police detain opposition leaders, break up protest
* Opposition blames Putin for stagnation, crackdown
By Gabriela Baczynska and Alissa de Carbonnel
MOSCOW, Dec 15 (Reuters) - Russian riot police detained four opposition leaders and broke up a crowd of about 2,000 people who went ahead with a banned rally on Saturday to demand an end to Vladimir Putin's 13-year rule.
The opposition chose a symbolic location, in front of the Soviet KGB security police's former headquarters, for the rally marking a year of protests against Putin, and said the police intervention showed the limits on dissent under the president.
Police were out in force and helicopters buzzed overhead as protesters, wrapped in scarves and fur hats because of the cold, chanted "Down with the police state" and "Russia without Putin" on the Lubyanka Square in central Moscow.
One unfurled a banner saying "crooks and thieves" - a popular term coined by bloggers for the Russian leadership.
The police eventually lost patience with the rally, which had been banned by Moscow city authorities, and strode across the square hauling protesters away one by one. About 40 people were detained, police said, and there were minor scuffles.
Leftist leader Sergei Udaltsov and anti-corruption blogger Alexei Navalny were detained at the start of the rally and two fellow protest leaders, Ilya Yashin and Ksenia Sobchak, were detained on their way to the protest.
All four were released without charge hours later, indicating they had been detained to prevent them stirring up the crowd at the protest.
"I don't know how many people are here but I am proud of each and every one of those who came here. The main thing is that people are here, that they are expressing their view and showing that they exist," Navalny said before he was detained.
"Obviously the authorities don't like attempts to carry out such protest actions and the development of the protest movement in general. They don't like anything that threatens them."
A YEAR OF PROTESTS
Protests began a year ago after Putin's United Russia party won a parliamentary election marred by allegations of vote-rigging, but quickly developed into the biggest movement against the former KGB spy since he first came to power in 2000.
At their peak last winter the biggest rallies attracted up to 100,000 people, witnesses said. But attendance has dwindled since Putin began a six-year third term as president in May and started what the opposition says is a clampdown on dissent.
"Not a single one of our demands has been met and the political repressions continue," said Vladimir Ryzhkov, one of the main protest leaders and a former member of parliament.
Despite the ban on the rally, protesters came out in temperatures of minus 15 Celsius (plus 5 Fahrenheit) to show their concern that Putin's return to the Kremlin is leading Russia into economic and political stagnation.
"I'm scared of arrest but I'm more scared that my children will want to live in another country," said Alexander Ivanov, 39, a businessman. "I'm afraid it's already too late. Putin and this country are incompatible. He's running it into the ground."
One protester, a translator who gave her name only as Anna, brought her prayer book with her.
"I'm praying for Russia. God made us free. No one can take that away from us, or punish, detain or torture us for our political views," she said.
ATTENDANCE FALLS AT PROTESTS
The 12 months of protests have accelerated the birth of a civil society two decades after the collapse of the Soviet Union but the opposition - a disparate group of leftists, liberals, nationalists and ecologists - broadly acknowledges it must now hope for political evolution rather than revolution.
The protests failed to prevent Putin, now 60, winning a presidential election in March after four years as prime minister. He has a grip on state media, retains support in the industrial and provincial heartlands that have long been his power base and could rule until 2024 if re-elected in 2018.
"Fewer and fewer people are going to the protests. It's fading because I don't see any leaders for me here," said Yelena, 45, an engineer who was afraid to give her last name.
But she added: "I am here out of solidarity with the people. We came because we are unhappy with the way things are going."
New laws broaden the definition of treason, increase punishment for protesters who step out of line, and tighten control on lobby groups that receive foreign funding.