* Law extending definition of treason goes into force
* Critics say it can be used to stifle dissent
* FSB says law needed to deal with foreign intelligence
By Nastassia Astrasheuskaya and Steve Gutterman
MOSCOW, Nov 14 (Reuters) - Russia enacted a new treason law on Wednesday, alarming opponents of President Vladimir Putin who fear he will use it to silence critics and - in a reminder of the Soviet past - put at risk almost anyone who associates with foreigners.
The law broadens the definition of treason to allow Russians representing international organisations to face the charge, as well as citizens working for foreign states and bodies.
Putin signed the legislation on Tuesday which took effect on Wednesday when it was published in the official gazette, Rossiyskaya Gazeta, even though he had promised on Monday to review it.
Political opponents and rights activists say the law is the latest in a series intended to crack down on the opposition and reduce foreign influence which have been introduced since Putin returned to the Kremlin in May for a six-year third term.
Lyudmila Alexeyeva, 85, a former Soviet dissident and veteran human rights activist, drew comparisons with the darkest days under dictator Josef Stalin, who died in 1953.
"It's an attempt to return not just to Soviet times but to the Stalin era, when any conversation with a foreigner was seen as a potential threat to the state," she said, adding that the law would probably be used selectively against Kremlin critics and others "who irritate the authorities".
Dmitry Oreshkin, a political analyst sympathetic with anti-Putin protests this year, said the motivation behind the law was that "the state is more important than its citizens, so there must be as much control over citizens as possible".
The law was backed by the Federal Security Service (FSB), the main successor of the Soviet KGB secret police. It landed on the desk of Putin, who was once a KGB officer, after being approved by both houses of parliament in nine days last month.
The FSB, in a rare public comment, was quoted by state-run news agency Itar-Tass as saying the law had been unchanged since the 1960s and needed updating as "foreign intelligence agencies' methods and tactics for gathering information have changed".
Putin attacked the United States during campaigning for the presidential election in March, and Russian officials have said the law is needed to help prevent foreign governments using organisations in Russia to gather state secrets.
"Citizens recruited by international organisations acting against the country's interests will also be considered traitors," Rossiyskaya Gazeta said in a commentary on its website.
Putin has frequently accused Western nations of trying to undermine Russia's security. He has suggested they are using non-governmental organisations to weaken the nuclear-armed state, which was a Cold War enemy of the United States,
Moscow ordered the U.S. Agency for International Development (USAID) to cease its operations in Russia in October, accusing it of seeking to influence elections.
In July, Putin signed a law requiring foreign-funded NGOs deemed to be engaging in political activity to register as "foreign agents". His critics say other legislation is also aimed at silencing opponents.
The United States and the European Union have criticised the laws, and expressed concern about criminal charges laid against several opposition leaders in the last few months.
During his election campaign, Putin faced protests which at times drew tens of thousands of people into Moscow's streets, and he accused Washington of encouraging the demonstrators.
The maximum sentence for high treason remains 20 years, but the new legislation introduces prison terms of up to eight years for Russians acquiring state secrets in certain ways, even if they are not passed to foreigners.
It broadens the actions that can attract treason charges to include giving "financial, material, technical, consultative or other aid" to a government or organisation deemed to be seeking to undermine Russian security.
Those changes, as well as the removal of the stipulation that actions must be aimed against Russia's "external" security to be considered treasonous, have raised concerns the law could be applied broadly to punish government opponents.
At a meeting of his human rights council on Monday, Putin listened to a retired Constitutional Court judge's concerns about the legislation, which she said did not require authorities to prove a suspect had damaged state security.
But although Putin said he would look again at the law, his spokesman, Dmitry Peskov, said he signed it a day later.
"It's not the first time Putin has said the right words while slowly tightening the screws," Alexeyeva said.
Peskov said Putin could still seek to amend the law if it was found in practice to curtail citizens' rights, but critics were sceptical. "It's high time to understand one should not pay attention to what Putin says," said Dmitry Oreshkin, an opposition-minded political analyst.
Putin was increasingly turning to "Soviet values" that put the state above the rights of citizens to maintain his grip, he said. "He could not fail to sign it because the FSB would tell him: 'In that case, we cannot ensure control'," Oreshkin said.