BERLIN - The discovery of an unexploded World War II bomb unearthed near Berlin's main train station snarled rail and road traffic in the German capital Wednesday, authorities said.

"The bomb was found yesterday (Tuesday) afternoon on property belonging to the rail company," a police spokesman told AFP, adding that sappers planned to defuse it Wednesday.

The 100-kilogramme (220-pound) explosive was dropped by an Allied plane during the Second World War, he said. Media reports said it was a Soviet bomb.

The site lies about 1.5 kilometres (0.9 miles) north of the main station and the rail operator, Deutsche Bahn, said up to 50 regional and long-distance trains had to be re-directed to other hubs from 0800 GMT Wednesday.

The changed travel plan was expected to last three hours, with suburban train links unaffected and the main station itself still open.

 Some 3,000 bombs beneath Berlin

Residents in the immediate area were ordered to evacuate ahead of the defusing operation planned for around midday, streets were cordoned off and ship traffic on a neighbouring canal was stopped.

"The detonator will be unscrewed with a tool," the police spokesman said.

"The sappers are trained but if something goes wrong they're history because they're directly next to the bomb."

More than 6 decades after the war, authorities believe there are still some 3,000 bombs buried beneath Berlin alone, and unexploded devices are regularly discovered in construction work.

In June 2010, a 500-kilo Allied bomb, thought to be British, exploded and killed three German sappers, and seriously injured two others in the central city of Goettingen as they prepared to defuse it.