One of the pilots died at the scene Thursday and five others were injured, fire service spokesman Stephan Fleischer said.

It wasn't immediately clear if the injured people were in the helicopter, Fleischer added.

Two of the injured - a man and a woman - were in serious condition, police said.

Watch video 01:07 Helicopters crash in Berlin

Witnesses said the crash took place at around 10:30 a.m. local time during a snow shower, and pieces of the wreckage could be seen lying around the site of the accident.

The exact cause of the accident is unclear.

Footage from German television broadcaster n-tv showed one helicopter on its side on the ground and the other next to it, still upright.

An eyewitness identified as Johannes M. told n-tv he saw three helicopters approaching and snow on the field was being kicked up by the aircraft, making it difficult to see.

Then "there was a big bang and someone shouted, 'everybody on the ground,'" he said. "And then we looked up and there was blood on the ground."

The police were conducting simulated action against football hooligans. About 400 officers were involved in the exercise.

The Olympic Stadium was home to the 1936 Summer Olympics and the 2006 World Cup Final. It currently hosts the annual German Cup Final and serves as the home ground for Bundesliga side Hertha Berlin.

dr/jlw (AP, dpa, SID, Reuters)