Speaking after the hearing, Mrs Litvinenko said she hoped to end speculation surrounding her husband's death.
Litvinenko's wife Marina and their 12-year-old son Anatoly were among the mourners, PA said.
British government documents that implied Russia was behind Mr Litvinenko's death have also been revealed.
Last month, he fainted while attending a conference in Ireland, on the day after Litvinenko died.
Mr Litvinenko died in 2006 after he was apparently poisoned with the radioactive substance polonium-210.
BBC: Alexander Litvinenko murder was 'London nuclear terror'
Russia has not yet commented on the claims of evidence of its involvement in Litvinenko's death.
Russia did not immediately comment on the claims of evidence of its involvement in Litvinenko's death.
Another version has it that Mr Litvinenko was poisoned by someone in Russia with a grudge.
All this without Britain's accusations of Russian state involvement in the poisoning of Mr Litvinenko.
Litvinenko's father Valter, his mother Nina Belyavskaya and his first wife Natalia were also at the ceremony.
It was due to formally open on May 1, more than six years after Mr Litvinenko died.
The prime suspect for Mr Litvinenko's death is ex-KGB officer Andrei Lugovoi who was represented in court.
BBC: Alexander Litvinenko murder was 'London nuclear terror'
The matter is of interest to Britain because it gave Litvinenko asylum in 2000 and granted him citizenship.
Mario Scaramella told the Associated Press he is cooperating with authorities for investigating the death of Alexander Litvinenko.
Litvinenko had an MI6 handler known only as "Martin, " whom he would meet in central London, he said.
For years, Berezovsky bankrolled the effort of Litvinenko's widow to push for an inquest into her husband's death.
Mr Davies told the hearing there were a "number of competing and increasingly controversial theories" surrounding Mr Litvinenko's death.
The confidential government material does not, however, support the idea that the British government itself killed Litvinenko, he said.
The British government has evidence that Russia was involved in Litvinenko's death, a British lawyer said in December.
Mikhail Trepashkin, a former counterintelligence colleague of Litvinenko who once warned the ex-spy of a plot to kill him.
Scaramella served with Litvinenko on an Italian parliamentary probe of alleged links between Italian politicians and the Soviet-era KGB.
Mr Litvinenko, 43, was poisoned in London in 2006 - allegedly at a meeting with two Russian former KGB contacts.
Four British detectives are in Moscow as an investigation of the poisoning death of former KGB officer Alexander Litvinenko widens.
Lugovoi met Litvinenko in London on November 1, 2006, at the Millennium Hotel bar, hours before the 43-year-old fell ill.
Others with interested person status include Home Secretary Theresa May, Mrs Litvinenko and her son Anatoli, and Russian tycoon Boris Berezvosky.
The PII application is being opposed by lawyers for Mr Litvinenko's widow, Marina, as well as media organisations, including the BBC.
He said he sought out Litvinenko's opinion during a business trip to London because of his prior contacts with the source.
The committee, a federal agency, is carrying out its own investigation into the circumstances of Litvinenko's death in Russia, the statement said.
It was an early suspect in the race to determine what killed former KGB agent Alexander Litvinenko in London two years ago.
British police have concluded an investigation into the death of Alexander Litvinenko.
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