The officers died in the Barents Sea on Monday, but the accident was only made public a day later.

Moscow has said the crew was studying the sea floor and that the details of the tragedy are a "state secret".

But Russian media have reported that the ship was a top-secret nuclear-powered mini-submarine.

The defence ministry confirmed the vessel was nuclear-powered for the first time on Friday.

The funerals, which were closed to the media, took place in the former imperial capital's historic Serafimovskoye Cemetery.

They began amid heavy security, with military police standing outside the cemetery gates.

"You have to understand that the identities of most of the people who gathered here are secret and their faces cannot be shown," a representative of the defence ministry told AFP.

An AFP reporter saw 14 vehicles carrying coffins drive into the cemetery. Later, a military salute and the Russian national anthem were heard.

Dmitry Razmyslov, 38, attended the funeral to pay his respects to one of the seamen -- Dmitry Solovyev -- with whom he studied at a military academy in the late 1980s.

"I did not know the details of his job. It was secret," Razymyslov, who did not go on to have a military career, told AFP.

The Russian defence ministry said Solovyev acted heroically during the accident, evacuating a civilian and closing the vessel's hatch to halt spreading flames.

"He was very responsible and, despite the fact that his wife is expecting a child, he acted the way he did," Razmyslov said.

- 'Real heroes' -

"This is a great sorrow," said a young woman clad in black attending the ceremony. She held a wreath that read "from friends and classmates".

Some ordinary Saint Petersburg residents came to the cemetery's gates to pay their respects.

"I feel so sorry for them, like they were my own (family)," said 60-year old Natalya Stepanova, who lives nearby.

"They are real heroes," she said.

Russian President Vladimir Putin posthumously awarded state honours to all 14 of the submariners.

The tragedy has echoes of the sinking of the Kursk submarine in 2000, also in the Barents Sea, that claimed 118 lives and shook the first year of Putin's presidency.

Russian media reported that the 14 seamen will be buried near a monument to the Kursk victims in the Serafimovskoye Cemetery.

On Friday, the defence ministry said the fire started in the submersible's battery compartment and did not impact the vessel's nuclear reactor.

Nearly all of the victims were highly decorated officers and included seven Captain First Rank officers -- the most senior staff officers in the Russian navy.

Experts have said that the presence of many high-ranking officers on board could suggest the submarine was not on an ordinary assignment.