Moscow (CNN) -- At least 55 people are now known to have died when a ship sank with more than 200 people aboard on Russia's Volga River, state-run media reported Monday, citing the Russian Emergency Situations Ministry.
Dozens of people are still missing after the ship "Bulgaria" went down on Sunday, the ministry said. Divers have been unable to reach parts of the boat where the largest number of people had congregated shortly before the accident, the state-run RIA Novosti news agency reported.
Many children are reported to have been aboard the pleasure cruise when the ship sank in the Russian republic of Tatarstan.
At least 79 people have been rescued, some of whom have been released from hospitals after receiving medical treatment, according to government agencies.
The ship did not have a license to transport passengers, it was overloaded, and was last repaired more than 30 years ago, the Russian Prosecutor's Office said Monday.
Prosecutors also established that the left engine of the ship was damaged, they said on their website.
Russian state TV reported the vessel had an operational limit of about 150 passengers, citing a top government official on the accident site. State TV also reported, citing law enforcement officials, that there were life vests on board for only 156 people.
Russian Emergency Situations Minister Sergei Shoigu said that according to the latest information, there were 208 people aboard the ship, of whom 25 passengers were not officially registered and didn't have tickets.
President Dmitry Medvedev is declaring Tuesday a day of national mourning for the victims, he said in an emergency government meeting Monday.
The president appointed Transport Minister Igor Levitin to head a special government commission to investigate the case.
"It is clear that such an accident couldn't have taken place if safety rules were followed, even despite the difficult weather situation," Medvedev said. "We have to establish why the owner of the ship operated a ship that was in such a poor technical condition."
He also called for "a total inspection of all public carriers in Russia," he said, adding that it is "obvious that this ship was not the only one with issues.
"The number of old tubs that are now in use (in Russia) is just staggering," he said.
"We can see from the information we have that the vessel was not in the appropriate condition," Medvedev said of the Bulgaria.
He instructed the Prosecutor General's Office to investigate everyone involved in the Bulgaria cruise, including "ship-owners, those who issued the navigation permit and those who were involved in organizing that boat tour, especially given the large number of children aboard."
Officials are making arrangements to lift the ship from the river bed, he said, saying that planning for the project will be completed later on Monday.
More than 80 scuba divers are now working on the site of the accident, with more arriving by the end of the day, the ministry said.
The search-and rescue operation is expected to be completed by Monday evening, the Emergency Situations Ministry regional office said on its website.
It is not clear how many people are still missing.
Authorities have not ruled out the possibility that some passengers could have survived by swimming to the bank or one of the islands in the Volga.
Survivors told Russian state TV that there were many children on board the ship.
The ferry was built in 1955 in the then-Czechoslovakia, according to Russian law enforcement officials.
RIA Novosti said the double-deck cruise ship went down near the village of Syukeyevo in Tatarstan, about 50 miles south of the city of Kazan. Kazan is about 450 miles east of Moscow.
Svetlana Alekseeva, a crew member of the ship Arabella, which picked up many of the survivors, spoke to the Russian news agency Interfax after the accident.
"It all happened so fast. The crew did not have time to pull out the lifeboats and were able to lower only two inflatable rafts. Many passengers simply jumped into the water. Few escaped from the chaos in the water, I'm afraid," said Alekseeva, Interfax reported.
CNN's Arkady Irshenko contributed to this report.