At least one police officer has been killed and four others injured after two helicopters collided at Berlin's iconic Olympic Stadium during a security exercise, authorities have said.

Police said on Thursday the dead officer was a pilot and that four police had been hurt, two of them - a man and a woman - seriously.

About 400 federal policemen were conducting an exercise to prepare for operations against violent football fans.

The Berlin fire brigade said in a statement that the two aircrafts crashed into each other at low altitude over a field on the stadium's grounds at about 09:30 GMT during a spring snowstorm.

The stadium was the venue for the 1936 Games where four gold medals for African-American athlete Jesse Owens ruined Adolf Hitler's dreams of Aryan glory for the Nazi hosts.

It is now the home stadium of Berlin football team Hertha BSC and served as the venue for the 2006 World Cup final.

'Pool of blood'

The daily Bild said on its website that half the officers were playing hooligans, some of them setting off fireworks to mimic a riot situation.

Three helicopters were used in the operation, the third of which landed without incident.

Eyewitnesses said the choppers' rotors had stirred up snow from the ground as flurries continued from the sky, further limiting visibility.

"It was a real squall - suddenly we heard a bang and someone shouted 'Everyone on the ground!'," a witness identified only as Johannes M told the NTV rolling news channel. "There was blood everywhere. There was an enormous pool of blood next to me."

Bild said panic broke out among the survivors, many of whom raced to flee the site. Police on the scene cared for the wounded next to the smoking wrecks of helicopters.

Source: Agencies