Up to 70,000 people are to be evacuated from the Greek city of Thessaloniki ahead of an operation to defuse an unexploded second world war bomb.

The bomb, containing nearly 250kg (550lb) of explosives, was unearthed in the northern city during roadworks last week and is due to be defused on Sunday.

More than 300 disabled people and bedbound patients were to be the first evacuated on Saturday using 20 ambulances, authorities from Greece’s second city said.

The full evacuation of all residents within a 1.1-mile radius of the bomb site is due to be completed before 8am on Sunday.

Evacuation was “obligatory”, the regional security chief Apostolos Tzitzikostas said. He added that the operation was unprecedented in Greece, “where a bomb of this size has never been found in an area this densely populated”.

Regional authorities said the entire operation would take up to eight hours.

A local military spokesman, 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. It was not known who dropped the bomb or when it fell.

A thousand police officers have been mobilised for the evacuation, with residents given several days’ warning via the media, leaflets and posts on social networks. Thessaloniki residents were facing disruption on the bus and train networks, with facilities set up to host evacuees in need of shelter.

People living in a nearby refugee camp will also have to be evacuated, the migration ministry said, without specifying the number affected. At their request, the refugees will use the evacuation as an opportunity to visit the local archaeology museum, the ministry added.

Seven decades after the end of the second world war, unexploded bombs are still being found around the globe. On 23 January, dozens of people were evacuated after a bomb was found near a Hong Kong university. Three days earlier, experts disposed of a suspected second world war bomb found close to the Palace of Westminster in London.

In the German city of Augsburg, 54,000 people had to leave their homes on 25 December while authorities dealt with a bomb dropped by Britain during the second world war.