Putin, in Finland, offered his condolences for the death of Alexander Litvinenko and denied any involvement. He called the release of the deathbed statement a "political provocation" by his opponents.
Litvinenko died late Thursday at a London hospital after spending days in intensive care as doctors puzzled over what was causing his organs to fail and attacking his bone marrow and destroying his immune system.
Britain's Health Protection Agency said Friday that the radioactive element polonium-210 had been found in his urine, and the police said traces of radiation were found at Litvinenko's home and a ritzy hotel bar and sushi restaurant he visited on the day he became ill.
Police said they were treating the case as an "unexplained death" - but not yet as a murder.
The 43-year-old Litvinenko, who fiercely criticized Putin's government from his refuge in London since 2000, told police he believed he was poisoned Nov. 1 while investigating the October slaying of Russian journalist Anna Politkovskaya, another critic of Putin.
Litvinenko's statement, read by his friend Alex Goldfarb to reporters outside the hospital, put the blame for his death squarely on Putin.
He accused Putin of having "no respect for life, liberty or any civilized value."
"You may succeed in silencing me but that silence comes at a price. You have shown yourself to be as barbaric and ruthless as your most hostile critics have claimed," the statement said.
"You may succeed in silencing one man but the howl of protest from around the world will reverberate, Mr. Putin, in your ears for the rest of your life."
Goldfarb said Litvinenko dictated the statement before he lost consciousness Tuesday, and signed it in the presence of his wife, Marina.
Putin strongly denied involvement by his government.
"A death of a man is always a tragedy and I deplore this," the Russian leader said when asked about Litvinenko during a news conference after a meeting with European Union leaders.
Putin said the fact that Litvinenko's statement was released only after his death showed it was a provocation. "It's extremely regrettable that such a tragic event as death is being used for political provocation," he said.
At a meeting Friday with Russian Ambassador Yury Fedotov at London's Foreign Office, British diplomats asked Moscow to provide all assistance necessary to a police inquiry into the death, government officials said. Putin pledged to cooperate.
Home Secretary John Reid convened the British government's crisis committee Friday to discuss the death, a Cabinet Office spokeswoman said. Prime Minister Tony Blair's Downing Street office said he was in Scotland and did not attend.
In Washington, State Department spokesman Tom Casey said the United States has sought information on the case from British authorities. "We have been told that they have no definitive conclusions and that they are conducting an investigation," Casey said.
The Health Protection Agency described poisoning with polonium-210 as "an unprecedented event."
"I've been in radiation sciences for 30-odd years and I'm not aware of any such incident," said Roger Cox, director of the agency's center for radiation, chemicals and environmental hazards.
The agency's chief executive, Pat Troop, said the high level of polonium-210 indicated Litvinenko "would either have to have eaten it, inhaled it or taken it in through a wound."
Troop said the agency was evaluating whether it was safe to perform an autopsy.
Peter Clarke, head of London's anti-terrorist police, said officers and military radiation experts were searching several locations in London.
Traces of radiation had been found at Litvinenko's north London house, the sushi restaurant where he met a contact Nov. 1 and a hotel he visited earlier that day, Clarke said. The restaurant and part of the hotel were closed, with officers removing materials in heavy metal boxes.
Clarke said extensive tests by forensic toxicologists on behalf of police - which began before Litvinenko's death - had on Friday confirmed the presence of polonium-210.
"There is no risk to the public unless they came into close contact with the men or their meals," said Katherine Lewis, a spokeswoman for the Health Protection Agency.
Experts said small amounts of polonium-210 - but not enough to kill someone - are used legitimately in Britain and elsewhere for industrial purposes.
Professor Dudley Goodhead, a radiation expert at the Medical Research Council, said that "to poison someone, much larger amounts are required and this would have to be manmade, perhaps from a particle accelerator or a nuclear reactor."
Chris Lloyd, a British radiation protection adviser, said it would be relatively easy to smuggle polonium into the country, because its alpha radiation would not set off radiation detectors.
Doctors treating Litvinenko had said Thursday that they could not explain his rapid decline. They discounted earlier theories that the father of three had been poisoned with the toxic metal thallium.
Lewis, the Health Protection Agency spokeswoman, said doctors had not discovered the presence of polonium-210 in Litvinenko earlier because hospitals do not normally test for the alpha-ray radiation it emits.
University College Hospital, where Litvinenko died, said Friday it could not comment further because the case was being investigated by police.
Litvinenko's friends had little doubt about who was to blame.
They said the former spy, who sought asylum in Britain in 2000 and became a citizen, worked tirelessly to uncover corruption in Russia's Federal Security Service, the successor agency to the Soviet-era KGB, and unmask Politkovskaya's killers.
Litvinenko worked for the KGB and the Federal Security Service. In 1998, he publicly accused his superiors of ordering him to kill Russian tycoon Boris Berezovsky and spent nine months in jail on charges of abuse of office. He was later acquitted and moved to Britain.
Litvinenko's father, Walter, said his son "fought this regime, and this regime got him."
"It was an excruciating death and he was taking it as a real man," Walter Litvinenko told reporters outside the hospital, his voice choked with emotion.
Goldfarb said the attack bore "all the hallmarks of a very professional, sophisticated and specialist operation."
Another friend, Andrei Nekrasov, said Litvinenko told him: "The bastards got me, but they won't get everybody."