Athens, Feb 12 (IANS): Nearly 72,000 residents in the northern Greek city of Thessaloniki were evacuated on Sunday morning ahead of a massive operation to defuse a WW II bomb, authorities said.

The 250-kg bomb, believed to be dropped by either British or US forces in 1943-1944 targeting Nazi German facilities in Greece's second-largest city, was discovered during excavation works at a gas station in Kordelio-Evosmos district a few days earlier, Xinhua news agency reported.

As a precautionary measure, while a Greek military bomb disposal squad will be working in the area for some six to eight hours according to initial estimates, authorities decided to proceed to the biggest such evacuation operation in post-war Greece.

City authorities have been instructing -- through leaflets and media during the past few days -- all residents living within two km around the gas station to leave their homes by 10 a.m.

Buses were available to transfer the affected to schools, stadiums and cafes early on Sunday, while bed-ridden patients were transferred on Saturday.

Refugees and migrants living in nearby accommodation camps were also temporarily evacuated.

Around 1,000 security personnel have been deployed to the area, the police said. They knocked on doors to make sure the houses were empty before experts start to defuse the detonator.

According to the army, the squad has disposed of dozens of similar bombs. However, it was the first time in decades a bomb of such a size was found in a densely populated urban district in Greece.

According to the plan, once the detonator is defused, the bomb will be transferred to an army base located about 30 km away.

Authorities have sealed off the route. During the operation, train services and traffic will be halted.