Media reports said he had been poisoned with the deadly chemical thallium which is used in rat poisons and insecticides.
"I feel very bad. I've never felt like this before - like my life is hanging on the ropes," Litvinenko said from his hospital bed in London.
Alex Goldfarb, a close friend, told Al Jazeera: "He looks like a ghost. He has lost all his hair and looks like a very old man. Before the poisoning he looked like a 44-year-old healthy man.
"The doctors say that the next four weeks are critical. His chances for survival are 50-50."
Police refused to go into details but a spokesperson said: "Officers from the specialist crime directorate are investigating a suspicious poisoning. No arrests have been made. Inquiries are continuing."
Litvinenko, who is regarded as a traitor in Russia after he defected six years ago with his family, went to meet his contact at the Sushi restaurant believing that he would recieve information on the killing of Anna Politkovskaya.
Litvinenko, a a former KGB and [Russian] Federal Security Service agent, said that he had been handed a document with information connected to Politkovskaya.
"It contained a list of names of people, including FSB officers, who were purported to be connected with the journalist's murder," he said.
Litvinenko's 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 his 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 is [Russian President Vladimir] Putin's enemy, he started to criticise him and had lots of fears."
Akhmed Zakaev, the Chechen foreign minister, told Al Jazeera: "After the assassination of former Chechen president, Russia is clearly exporting the methods of state it uses in Chechnya to other countries."
There have been a number of other high-profile cases of poisoning that are thought to have been carried out by Russia. The most recent being that of Victor Yushenko, the Ukrainian president, who was poisoned by political rivals with links to Russia.
Thallium attacks the nervous system lungs, heart, liver and kidneys. The colourless, odourless toxin results in hair loss, vomiting and diarrhoea. One gram can be enough to kill