A massive evacuation has begun in the northern Greek city of Thessaloniki so experts can defuse a 500-pound unexploded World War II bomb.

An estimated 75,000 people are being evacuated in a nearly 2-kilometer (1.2-mile) radius Sunday. The bomb, dropped during an air raid in the 1940s, was found this month during work to expand storage tanks under a gas station.

A state of emergency has been declared in the three municipalities involved and about 1,000 police and 300 volunteers helping out. Trains have been halted and church services cancelled.

Thessaloniki's Deputy Governor Voula Patoulidou told The Associated Press "it's the first time something like this is happening in Greece."

Residents are being bused to schools, sports halls and cultural centers during the operation, which is expected to take six hours.