Mr Litvinenko's condition deteriorated rapidly in hospital
Litvinenko statement
Mr Litvinenko, 43, who died in a London hospital on Thursday evening and is thought to have been poisoned, said his killer was "barbaric and ruthless".
Protest from around the world "will reverberate, Mr Putin, in your ears for the rest of your life," he said.
The Kremlin has dismissed allegations it was involved as "sheer nonsense".
Scotland Yard said officers were now investigating "an unexplained death".
Anti-terror police are leading the investigation, and it is still unclear what killed the former KGB agent.
Friends have said he was poisoned because of his criticism of Russia.
'Barbaric and ruthless'
In the statement, read out by his friend Alex Goldfarb outside University College Hospital, London, Mr Litvinenko said he had a "message to the person responsible for my present condition".
LITVINENKO TIMELINE 1 Nov - Alexander Litvinenko meets two Russian men at a London hotel and also meets Italian academic Mario Scaramella at a sushi bar in Piccadilly. Hours later he falls ill and is admitted to Barnet General Hospital 17 Nov - Mr Litvinenko is transferred to UCH 19 Nov - Reports say Mr Litvinenko is poisoned with thallium 21 Nov - A toxicologist says he may have been poisoned with "radioactive thallium" 22 Nov - Mr Litvinenko's condition deteriorates overnight. Thallium and radiation ruled out 23 Nov - The ex-spy dies in intensive care
Litvinenko statement in full Reaction: Russian's death Timeline of case in full
"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 howl of protest from around the world will reverberate Mr Putin in your ears for the rest of your life," the statement added.
The statement was dictated on 21 November, when Mr Litvinenko realised he could die.
Mr Putin's spokesman Dmitry Peskov reiterated the Kremlin's earlier dismissal of allegations of involvement in the poisoning as sheer nonsense.
"Any death is always a tragedy," he said.
"Now it's up to the UK law enforcement agencies to investigate what happened."
'Excruciating death'
After Mr Goldfarb had read out the statement, Mr Litvinenko's elderly father, Walter - who flew to the UK from Russia this week - said his son had been killed by a "tiny nuclear bomb".
Even before his death, in such a state, he never lost his human dignity
Walter Litvinenko
Obituary: Alexander Litvinenko
"It was an excruciating death, he was taking it as a real man," he said.
"Even before his death, in such a state, he never lost his human dignity."
Mr Litvinenko had recently been investigating the murder of his friend, Russian journalist Anna Politkovskaya, another critic of the Putin government.
Russian dissident Oleg Gordievsky, a former KGB colonel and friend of Mr Litvinenko, maintained that the poisoning had been the work of the Russians.
The Russian security service had "sent a man with a poisonous pill to Britain", put a pill into Mr Litvinenko's tea and killed him, he told BBC News.
Intelligence analyst Glenmore Trenear Harvey said Mr Litvinenko had "made a lot of enemies" when he had been tasked with fighting corruption during his time with the Federal Security Service (FSB) - the KGB's successor.
Mr Harvey also said the poisoning could have been carried out by the "Russian mafia", made up of former-KGB men who had formed the group when the service broke up.
"So I think that while one could say they were trained by the KGB this is not in any way a Russian intelligence service hit," he told BBC News.
London meetings
Before Mr Litvinenko's death, police said they suspected "deliberate poisoning" was behind his illness.
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Mr Litvinenko, who was granted asylum in the UK in 2000 after complaining of persecution in Russia, fell ill later that day.
In an interview with Friday's Telegraph newspaper, former KGB bodyguard Andrei Lugovoi said he had met Mr Litvinenko at the Millennium Hotel in Grosvenor Square but vigorously denied any involvement in the poisoning.
Mr Scaramella, who is involved in an Italian parliamentary inquiry into Russian secret service activity, said they met because he wanted to discuss an e-mail he had received.
Speaking in Friday's Times, film-maker Andrei Nekrasov said that, before he fell unconscious for the last time, his friend had told him: "I want to survive, just to show them. The bastards got me but they won't get everybody."
Mr Alexander Litvinenko, reputed to have been an Ex-Russian spy who had defected to Britain, died last night in mysterious circumstances. He had alleged many associations between people in high places and organised international crime, implying that President Putin and Romano Prodi among others had been involved personally. He had sought political asylum in UK in 2000 and became a naturalised citizen in October this year, just weeks before his death.

Mr Litvinenko was said to have been investigating the shooting of Russian journalist and human rights activist Anna Politkovskaya, a well-known critic of Russian activities in , in her apartment in Moscow October 7, 2006. It is reported that he had met two Russians in a hotel room, one said to be former member of the  in connection with the affair and he went on to meet Mario Scaramella at a  bar in Piccadilly where some papers were exchanged. Some hours after this he was taken ill. 

Mr Litvinenko was admitted to , north London on November 1, 2006 complaining of feeling sick. By November 11, he was said to be suffering from ''serious poisoning''. A week later he was transferred to  in central London. A week later he was said to have been poisoned by ingesting , once used in rat poison, but, in the opinion of some doctors, there were signs of , including loss of weight and shedding of hair. Various explanations of his condition were offered. Last night he suffered a heart attack, after having left a message blaming President Putin for having him killed. 

The radioactive  -210 was found in his blood and urine as reported by the . The post-mortem was cancelled. Subsequently the Agency examined conditions in the hospitals in which Mr Litvinenko has been treated. Police visited the Itsu sushi restaurant in Piccadilly, his home in Muswell Hill and the Millennium Hotel, Grosvenor Square where the meeting on November 1 had been held. Traces of radiocactivity were found in all three places. It is speculated that the polonium was probably eaten by Litvinenko as a substance that could be combined with a salt-like substance, such as .

It was reported that a meeting had been held in Cabinet Office Briefing Room A (COBRA) used for high level emergency planning and control, to consider the implications of these events. The Foreign Office asked Moscow for a response to the accusation of Russian involvement and President Putin himself dismissed the allegations saying (before the cause of death had been established) that there was no proof of an unnatural death and that the case was being used as a "political provocation".