MOSCOW, Feb 16 (Reuters) - Alexei Navalny, Russian President Vladimir Putin's most formidable domestic opponent, fell unconscious and died on Friday after a walk at the "Polar Wolf" Arctic penal colony where he was serving a three-decade sentence, authorities said.

The death of Navalny, a 47-year-old former lawyer, robs the disparate Russian opposition of its most prominent leader as Putin prepares for an election which will keep the former KGB spy in power until at least 2030.

U.S. President Joe Biden said he was outraged and joined other Western leaders in criticising Russia over Navalny's death, blaming it on "something that Putin and his thugs did."

Navalny rose to prominence more than a decade ago by documenting and speaking publicly about what he said was the vast corruption and opulence among the "crooks and thieves" running Putin's Russia.

There are no other opposition leaders in Russia of Navalny's stature. For some young urban Russians, Navalny had offered hope of an alternative future to Putin, who has served as Russia's paramount leader longer than anyone since Josef Stalin.

The Federal Penitentiary Service of the Yamalo-Nenets Autonomous District said in a statement that Navalny felt unwell after a walk at the IK-3 penal colony in Kharp, about 1,900 km (1,200 miles) northeast of Moscow.

He lost consciousness almost immediately and died shortly afterwards, it said, adding that resuscitation attempts failed.

Navalny's spokeswoman, Kira Yarmysh, said there was "almost no hope" that he was alive.

Navalny's wife, Yulia, told the Munich Security Conference she could not be sure her husband was dead because "Putin and his government... lie incessantly".

"But if this is true, I want Putin, his entire entourage, Putin’s friends, his government to know that they will bear responsibility for what they did to our country, to my family, to my husband," she said.

The Kremlin said Putin had been informed of Navalny's death. The 71-year-old former KGB spy was shown meeting workers at a factory in Chelyabinsk, in the Ural mountains. He said nothing in public about Navalny.

Western leaders paid tribute to Navalny's courage as a fighter for freedom. Some, without citing evidence, accused the Kremlin of murder and said Putin should be held accountable.

Kremlin spokesman Dmitry Peskov said the reaction of Western leaders to the death was unacceptable and "absolutely rabid".

"We don't know exactly what happened, but there is no doubt that the death of Navalny was a consequence of something that Putin and his thugs did," Biden said at the White House.

"Russian authorities are going to tell their own story," Biden said. "But make no mistake. Make no mistake: Putin is responsible for Navalny's death."

Photo from Reuters