As giant floating crane stands in its lifting position, as mourners bring flowers and candles at the bank of Danube river on Monday in Budapest. Attila Kisbenedek/AFP

Hungarian police on Tuesday began lifting a sightseeing vessel that sank in the river Danube in Budapest last month in an accident that has claimed 20 lives with eight people still missing.

The Mermaid tourist boat was carrying mostly South Korean holidaymakers when it capsized and sank seconds after colliding with a bigger river cruise boat on a busy stretch of the river on May 29.

Only seven people of the 35 people on board are known to have survived the accident, with the prospect of finding any more passengers alive seen as practically zero.

Those missing include a six-year-old girl as well as the Mermaid's Hungarian captain.

Divers have been unable to enter the submerged boat due to the strong current in a river swollen after weeks of rain.

The salvage operation has instead focused on fixing wire harnesses underneath and around the vessel to prepare it for hoisting by a crane mounted on a barge.

A police spokesman said on Monday that the lifting operation could take six hours, and will be phased to allow divers to search the vessel for any victims still inside.

Once out of the water and removed to a secure location the Mermaid will be examined by police who have launched a criminal probe into the cause of the accident.

The captain of the river cruise ship, the Viking Sigyn, has been arrested on suspicion of “endangering waterborne traffic resulting in multiple deaths.”

Agence France-Presse