LONDON (AP) — Thousands of people were evacuated from homes and businesses in the English city of Bath after a 500-pound (225-kilogram) unexploded World War II shell was found under a school playground.

Police advised residents to leave more than 1,000 properties in a 300 meter (328 yard) zone around the device, found during construction work on a disused playground at the Royal High School Bath on Thursday. Some spent the night at a local racecourse.

Chief Inspector Kevin Thatcher of Avon and Somerset Police said Friday that army explosives experts were working to make the shell safe.

The bomb squad was building a barrier around the device with sand before moving it outside the city where it can be exploded safely.

Police said some residents might not be able to return until Saturday.

Britain was heavily bombed by Germany's Luftwaffe during the war, and undetonated explosives are sometimes found during construction work.

Bath, a city of elegant Georgian buildings 110 miles (177 kilometers) west of London, was targeted in the "Bath Blitz" over several days in April 1942. Some 400 people were killed and 19,000 buildings damaged.