LONDON, Nov. 24 -- Former Russian spy Alexander Litvinenko was fatally poisoned by a radioactive substance, traces of which were found in his urine, at his home and at a London restaurant and hotel he visited the day he became ill, according to the British health department. It called the case "unprecedented" in Britain.

Authorities closed the restaurant and sealed off part of the hotel Friday as part of an emergency effort to trace the substance, polonium 210, and ensure that it does not harm other people. Litvinenko would have eaten, inhaled or received it through a wound, according to Pat Troop, chief executive of Britain's Health Protection Agency.

Coming after the mysterious poisoning of another prominent opponent of the Kremlin, Ukrainian politician Viktor Yushchenko, the death provoked accusations that Russia continues to use Cold War-style tactics to eliminate critics abroad. London was the scene of the 1978 assassination of a Bulgarian dissident who was killed by a jab from a umbrella tip bearing the toxin ricin.

Litvinenko blamed the Kremlin shortly before he died, according to friends and family members. "As I lie here I can distinctly hear the beating of wings of the angel of death," Litvinenko, 43, said in a dictated statement, according to friend Alex Goldfarb, who met reporters while accompanied by Litvinenko's tearful father.

"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," the statement said. "May God forgive you for what you have done, not only to me but to beloved Russia and its people."

Kremlin and Russian security service officials have denied any involvement in the poisoning of Litvinenko, who fled to Britain in 2000 after publicly accusing the security service of involvement in the bombings of two apartment buildings in Russia in 1999 in which 300 people died. Litvinenko had been investigating the murder last month of Russian journalist Anna Politkovskaya, another critic of the Putin government.

Litvinenko's death complicated a summit in Helsinki between Putin and European Union leaders. Speaking there, the Russian president called Litvinenko's death a tragedy, expressing his condolences and pledging his country's cooperation in the investigation.

Addressing allegations that he had ordered the assassination to silence a troublesome critic, Putin said, "I hope the British authorities won't fuel groundless political scandals." He added, "It is a great pity that even such tragic things as human death are used for political provocations."

Putin questioned why Litvinenko's deathbed note had not been made public before he died. Putin also said there was no official finding that Litvinenko had been murdered. "As I know, the medical certificate of British doctors does not indicate that he died a violent death," Putin said. "It does not say that. Hence there is no reason for such talk at all."

Troop, the British health chief, said no autopsy had been conducted. She did not say why, but the BBC reported that "the delay is believed to be over concerns about the health implications for those present at the examination."

Roger Cox, another health agency official, told reporters that radiation emitted by the polonium had been detected in Litvinenko's urine.

John Henry, a toxicologist who was asked by Litvinenko's family to look into the case and who examined Litvinenko before his death, said the type of polonium involved is "only found in government-controlled institutions." In an interview, Henry called polonium 210 an "extraordinary poison" that is lethal in doses so small, "you can lose it on the point of a pin."

Henry, who took part in the investigation of the 2004 poisoning of Ukraine's Yushchenko, then opposition leader and now the president, said that polonium 210 "kills cell by cell" and that once it is administered, there's "absolutely nothing" that can be done to save the exposed person.

Peter Clarke, head of Scotland Yard's anti-terrorism branch, which is leading the investigation, said police are studying closed-circuit television surveillance footage of the places in central London that Litvinenko visited Nov. 1, the day he became ill.

In Friday interviews with Russian news media, a Russian man who met with Litvinenko that day said he had been questioned by British officials in Moscow and denied involvement in the case.

Andrei Lugovoy, a former Russian security agent, said Litvinenko called him and invited him to a meeting to discuss business contacts. He said that he and two other men, including one named Dmitry Kovtun, met with Litvinenko at the Millennium Hotel on Grosvenor Square near the U.S. Embassy.

He said Litvinenko did not eat or drink anything during their 20- to 30-minute meeting. "I'm surprised by how hysterically some are trying to tie me to this," Lugovoy said.

Police said Friday night that part of that hotel as well as the Itsu sushi restaurant that Litvinenko visited had been closed while the investigation continued.

From London to Moscow, people were trying to sort out who stood to benefit from Litvinenko's death.

Litvinenko's supporters say Putin benefits by eliminating a fierce critic. Kremlin defenders say it is not Putin, but rather the Russian leader's enemies who gain. The fierce anti-Putin circle in London, including exiled Russian tycoon Boris Berezovsky, has seized on the poisoning as a way to smear Putin's reputation internationally, they said.

Nikolai Kovalyov, chairman of the veterans committee of the lower house of the Russian parliament, told the Interfax news agency: "Doubtlessly, it did not benefit Russia and its special services. . . . It is not in our interests at all."

Kovalyov, who once headed the Federal Security Service, the domestic successor to the KGB, added that other "defectors who did incomparably more harm to Russia than Litvinenko continue to live in the West safe and sound."

Finn reported from Moscow.