Divers searching a sunken tourist boat in Russia's Volga River, have seen about 50 bodies, most of them children, an emergency situations ministry spokeswoman said.

Yelena Smirnykh, ministry spokeswoman, told Reuters on Tuesday, that divers had not yet begun to remove bodies from the vessel's recreation room.

"By their visual estimates, the bodies of about 50 people are there. Most of them are children," said Smirnykh.

Survivors said that many children had gathered in the room shortly before the boat, the Bulgaria, sank on Sunday.

Russia is observing a day of mourning on Tuesday. Flowers and small candles began to fill the main embankment of the central Russian city of Kazan, a regional capital about 750 kilometers east of Moscow, where the boat was en route to from the town of Bulgar.

The confirmed death toll stands at 63 as of Tuesday morning, but 64 people remain missing and hopes are slim that any more survivors will be found.

The 55-year-old double-decker boat was carrying 208 people, a load about 75 per cent more than it was licensed to carry, when it sank in water about 20 metres deep late on Sunday, according to the emergency situations ministry.

But whether the overloading was a factor in the sinking remains undetermined. Investigators say survivors report that the boat was listing to starboard and having engine trouble as it left Bulgar.

Russian President Dmitry Medvedev has ordered the creation of a state commission to investigate the sinking.

The prosecutor assigned to Volga River issues said the ship was running with a "defunct" left engine and had pumped fuel to the right side, where "technical water" also was being stored, causing the Bulgaria to tilt, the Russian Interfax news agency reported.

He also said the carrier operating the Bulgaria was not licensed to conduct tours.

Al Jazeera's Neave Barker, reporting from Moscow, said that since the river was very wide and the ship was in a remote section of the waterway, it was unlikely that help arrived quickly.

River cruises on the Volga, which cuts through the heart of Russia and drains into the Caspian Sea, are popular among Russians as well as foreigners.

Accidents rarely lead to major fatalities. The last shipping accident recorded by Russian state media occurred in September 2010, when seven people were killed on a lake above the Arctic Circle.

It was unclear whether the Bulgaria, which state TV said had dozens of cabins and two restaurants, was on a one-day outing or a longer cruise.

Source: Al Jazeera and agencies