Athens - Authorities in Greece's second-biggest city, Thessaloniki, have begun evacuating about 70 000 residents ahead of an operation to defuse a 250kg unexploded World War II bomb. The device, dropped during an air raid on the northern city in the 1940s, was unearthed in a densely populated area last week during works to expand fuel storage tanks.Bomb disposal experts will attempt to tackle the device, found near a petrol station on Saturday.More than 300 disabled people and bed-bound patients were the first to move out on Saturday morning at the start of what is being described as the biggest peacetime evacuation in Greece's history. The departure of all residents within a 1.9km radius of the bomb site, affecting three neighbourhoods in the west of the city centre, is due to be completed before Sunday morning. Refugees living in a nearby camp will also have to be evacuated, the migration ministry said, without specifying how many. The operation is unprecedented in Greece, "where a bomb of this size has never been found in an area this densely populated," Tzitzikostas added.Regional authorities said the entire operation would take up to eight hours, but local military spokesperson Colonel Nikos Phanios was more cautious."We don't know what we're going to find," he said. Defusing the bomb and then moving it to a military shooting range "could take us up to two days", he added. Up to 1 000 police officers have been mobilised for the operation, with residents given several days' warning via the media, leaflets and posts on social media.