June 4 (UPI) — More than 20,000 people in Germany were evacuated as authorities worked Wednesday to defuse three huge unexploded bombs from World War II at a construction site in the center of the city.

A major incident was declared due to the danger from the bombs — two 2,200 lb devices and one of 1,100 lbs — with police making checks door-to-door in the Old Town and Deutz areas of Cologne after 20,500 people were ordered to leave and businesses, tourist attractions and stations were shuttered, the city said in a news release.

The 1,100-yard-wide exclusion zone covers the city’s UNESCO-listed 13th-century cathedral, 58 hotels, parts of the subway system, at least nine schools, day care centers, two retirement homes, a hospital, as well as city hall and many other sites.

“The evacuation is the largest measure since the end of the Second World War. Everyone involved hopes that the defusing can be completed in the course of Wednesday,” city authorities said.

“This is only possible if all those affected leave their homes or workplaces early and stay outside the evacuation area from the outset on that day. We ask you to be cooperative and follow our instructions so that the evacuation and defusing can proceed quickly and without danger.”

Officials told Sky News the measures could remain in force for some time if the effort to defuse the devices was unsuccessful and it became necessary to detonate them, as that would require a major operation to contain the blast.

All three bombs are American-made, but likely dropped by the Royal Air Force, which dropped around 1.5 million bombs in raids on Cologne between 1940 and 1945, some of them with as many as 1,000 aircraft, launched from bases in eastern England.

However, as many as 300,000 of the bombs did not explode, according to experts, causing frequent scares when they turn up during construction projects or in dredging of the River Rhine, which runs through the heart of the city.

Last year alone, more than 30 were discovered, forcing 17 evacuations affecting 36,000 people. The bombs were among around 2,000 that are found across Germany each year, according to the Smithsonian Magazine.

In 2021, four people were injured in Munich when a World War II bomb exploded during construction work near the main train station and more than 65,000 people were evacuated in Frankfurt in 2017 after a “Blockbuster” 1.4-ton British bomb was found near Goethe University. That device was safely defused.

German bombs are frequently discovered in Britain, which was heavily bombed by the Luftwaffe between 1940 and 1941 and in 1944, most recently last year when 10,000 people were evacuated after a large bomb was found in the yard of a suburban property in Plymouth.

Naval and army bomb disposal officers extracted the device and took it out to sea, where it was detonated.