Col. Alexander Litvinenko, a former KGB and Federal Security Service (FSB) agent, was under armed guard in University College Hospital, London. He was in a "serious but stable" condition, the hospital said.
"He is still very weak," friend Alexander Goldfarb told reporters outside the hospital. "He is in a fighting mood, though."'
Police said a specialist crime unit began an investigation on Friday into how Litvinenko may have been poisoned. No arrests had been made so far, said a Scotland Yard spokesman, speaking on condition of anonymity in line with force policy.
Litvinenko, who had been looking into the killing of Russian investigative journalist Anna Politkovskaya, told reporters earlier this week that he fell ill on Nov. 1 following a meal at a sushi restaurant with a contact who claimed to have details about the murder.
British news outlets, including Sky News and The Independent, identified the contact as Mario Scaramella, an Italian academic who has helped investigate KGB activity in Italy during the Cold War. Scaramella could not immediately be reached for comment.
Politkovskaya, who had written critically about abuses by Russian and pro-Moscow Chechen forces fighting separatists in Chechnya, was gunned down Oct. 7 inside her Moscow apartment building. Her attackers have not been found. Rights groups have said the killing underscores the risks faced by Russians who question or criticize the government.
A doctor treating Litvinenko told the British Broadcasting Corp. that tests showed he had been poisoned by thallium — a toxic metal found in rat poison.
"He's got a prospect of recovering, he has a prospect of dying," said Dr. John Henry, a clinical toxicologist who treated Ukrainian President Viktor Yushchenko when he was poisoned by dioxin during his 2004 presidential election campaign.
Henry told the BBC that thallium can cause damage to the nervous system and organ failure, and that just one gram can be lethal.
Friends visiting Litvinenko in hospital said they were shocked by his appearance.
"He looks like a ghost," said Goldfarb. "He's a very fit man, he never smoked, he never drank, he would run five miles a day, but now he has lost all his hair, he has inflammation in the throat, so he cannot swallow."
Litvinenko quit Russia for Britain six years ago and has been an outspoken critic of the Kremlin ever since.
In 2003 he wrote a book, "The FSB Blows Up Russia," accusing his country's secret service agency of staging apartment-house bombings in 1999 that killed more than 300 people in Russia and sparked the second war in Chechnya.
His friends have said they believe Russian authorities could be behind the poisoning. Moscow did not comment on the allegations.
Russian dissident and tycoon Boris Berezovsky, who was at Litvinenko's bedside on Friday, told The Associated Press he suspected Russia's intelligence services were behind the alleged assassination attempt.
"It's not complicated to say who fights against him," Berezovsky said in a telephone interview. "He's (Russian President Vladimir) Putin's enemy, he started to criticize him and had lots of fears."