Latvia's coast guard on Monday evacuated a stranded cruise ship of more than 650 people after tug boats failed to pull the luxury liner off an underwater sand bank in the Baltic Sea.

In a five-hour operation, 651 passengers -- mostly elderly Germans -- and 11 crew members were transferred from the 660-foot-long Mona Lisa onto two naval ships, coast guard officials said.

They were taken about 22 miles to Ventspils, a port city in northwestern Latvia, for onward travel to the capital, Riga.

The passengers and crew members left the ship down stepladders to the naval ships without incident, coast guard spokeswoman Liene Ulbina said.

She said that 322 crew members remain on board on the captain's orders as the salvage operation continues.

The 30,000-ton cruise ship ran aground early Sunday about 10 miles off Latvia's coast, as it was passing through the Irbe Strait between Latvia and the Estonian island of Saaremaa.

The Mona Lisa's captain, a Greek national, agreed to evacuate the ship Monday after unsuccessful efforts to free it from the sand bank.

Initially, rescuers pumped ballast water from the ship to lighten the load, and then on Monday they tried pumping out fuel. But they were unable to free the liner.

The Mona Lisa was on its way from Kiel, Germany, to Riga with 984 people on board when it ran aground, officials said.

Coast guard officials said the ship, which went into service in 1966, was not damaged and passengers were never at risk. The cause of the accident has not been determined.

The evacuated passengers would be taken by bus and train from Ventspils to Riga, some 100 miles to the east, said Sofija Galindoma, a spokeswoman for Ventspils town council. They will be given food and water, she added.

Prime Minister Ivars Godmanis was in Ventspils to help oversee the operation, his spokesman Krists Leiskalns said.

Coast guard chief Hermanis Cernovs told the Baltic News Service that the Mona Lisa's owner -- Germany-based Lord Nelson Seereisen -- has asked a Swedish company equipped with a powerful tugboat to help free the ship.