Friends of former Russian spy Alexander Litvinenko accused "evil forces" in Russia of being responsible for his death in London overnight, saying the poisoning of the ex-colonel was an act of revenge.
Russia said it was silly to suggest the Kremlin had orchestrated a plot against Litvinenko (43), a critic of Russian President Vladimir Putin.
Alexander Litvinenko in his hospital bed earlier this week
British doctors said Litvinenko had been poisoned but did not know the exact form. His death last night occurred on the eve of an EU-Russia summit in Helsinki at which Mr Putin is now likely to face media questions about the former spy that could overshadow the main agenda of how the two sides can improve ties.
"He was fighting against the evil forces in Russia, against the KGB, against the authorities which are suppressing democracy and liberal freedoms in Russia," Oleg Gordievsky, a friend of Litvinenko, told Sky television.
"He became a victim of ... revenge and malice of those forces in Russia," said Mr Gordievsky, also a former Russian agent who defected to Britain.
Litvinenko, who lost all his hair and suffered major organ failure, died in a London hospital's intensive care unit after being poisoned three weeks ago. One doctor said the identity of the poison may never be known.
Medical experts have ruled out earlier suggestions it was caused by a heavy metal such as thallium or by radiation.
British police said they were investigating what they called the "unexplained" death and that anti-terrorism officers were working on the case.
Former Russian spy Alexander Litvinenko
If Russia was found to have had a hand in the poisoning, there could be far-reaching diplomatic consequences. It would be the first such incident known to have taken place in the West since the Cold War.
"The bastards got me. But they won't get everybody," Litvinenko told friend and filmmaker Andrei Nekrasov before losing consciousness earlier this week. The comments were published in today's London Timesnewspaper.
Litvinenko, who fled to Britain in 2000 with his wife and son and was granted asylum, said he fell ill after meeting two Russians at a hotel. He had been investigating the killing of Russian journalist Anna Politkovskaya, also a critic of Mr Putin, who was gunned down at her Moscow flat on October 7th.
A source in the Russian delegation with Mr Putin in Helsinki told reporters: "It is a human tragedy. The man was poisoned.
"But the accusations towards the Kremlin are so unbelievable, they are too silly to be commented on by the president or anyone from the Russian side," said the source, asking not to be identified.
Litvinenko served in the KGB's counter-intelligence department and then the Federal Security Service's (FSB) highly secret organised crime group. The FSB is the main successor organisation to the Soviet KGB and deals with internal threats.
In 1998, he turned on his former comrades and said at a Moscow news conference - with men in masks who claimed to be Russian secret service men - that senior FSB officers had planned to murder Russian tycoon Boris Berezovsky. He was arrested several times by his former employers at the FSB but was freed by a court and charges were dropped.