LONDON (Reuters) - Alexander Litvinenko's widow is seeking a ruling from the European Court of Human Rights that the Russian state was complicit in poisoning the former security agent with radioactive polonium, her lawyer said on Friday.

Louise Christian said she had obtained expert evidence it was "highly likely" the polonium had come from Russia's Avangard plant, a state facility surrounded by tight security.

"We say that there is evidence ... of either active complicity or connivance by the Russian Federation in the murder of Alexander Litvinenko," Christian told a news conference on the first anniversary of Litvinenko's death.

The former KGB security officer became a Kremlin critic in exile, and was poisoned by polonium slipped to him in a cup of tea. He spent three weeks dying in agony in London.

Britain wants to prosecute another ex-KGB man, Andrei Lugovoy, for the murder. But Russia strongly denies state involvement in the killing and refuses to hand over Lugovoy because its constitution bars it from extraditing its own nationals. The case has badly damaged diplomatic relations.

Christian, representing widow Marina Litvinenko, said the alleged state complicity in murder contravened the European Convention on Human Rights, which states the right to life.

POISONED TEAPOT

She said an independent nuclear expert -- whom she did not name -- had testified that British authorities should be able to trace the polonium's origin by comparing samples from the poisoned teapot with batches produced at Avangard and exported by Russia to various countries.

Russian businessman Boris Berezovsky, a friend of Litvinenko and Kremlin opponent whom Russia has tried unsuccessfully to extradite from Britain, told reporters: "I'm certain that Scotland Yard (police) and the British investigators know the origin of the polonium.

"Today it's hidden in the materials at the disposal of Scotland Yard. But Marina has the right, in the end, to demand an inquest, and then those materials will become public."

A police spokeswoman declined to comment.

Marina Litvinenko pledged to keep fighting for justice for her husband. "I promise one day we definitely will know who's responsible for this, because without this knowledge we just can't feel we are safe," she said.

Earlier, she joined Litvinenko's friends and father Walter for a short commemoration in front of the London hospital where he died. Dressed in black and wearing dark glasses, she listened as his last statement, accusing President Vladimir Putin of ordering his death, was read out just as it had been a year ago.

Walter Litvinenko branded Putin an "executioner". "I hope my son didn't die in vain and this villain will answer for everything," he told reporters.

Christian said she had filed Marina's case with the European Court of Human Rights on Thursday.

"They can award compensation, but clearly that's not the main aim of this," she told Reuters. "The main aim is to force them (Russia) to take responsibility and put things right ... The minimum would be an effective investigation."