By Ray Furlong
BBC News, Berlin
With a wail of sirens, a bright orange ambulance forged its way through a throng of panic-stricken football fans. Injured people were being carried over to it, blood streaming from their faces.
Germany wants to make the World Cup as secure as possible
As part of a mock disaster scenario being played out at a snow-covered field in south Berlin, 20 people had been killed and 200 injured when a big screen collapsed during a World Cup match.
"I think it's really realistic. There are people running around, it's very chaotic, and it's our job to bring order to the chaos," said police spokesman Kai Nolle.
"We learn how to co-ordinate and co-operate with all rescue forces. It's the German Red Cross, the fire brigade, and the Berlin police all working together."
This scenario was just one small part of Operation Triangle. Across town, there was a simulated chemical leak at a railway station - while an explosion at an old hospital was the third mock accident.
The German authorities say anything could happen during the World Cup, which runs from 9 June until 9 July, when the final is played in Berlin's Olympic Stadium.
"Maybe we will have an attack. Look back to Madrid, look back to London, and then you can see three things can happen at the same time," said Ehrhart Koerting, Berlin's chief security official.
"So we must be prepared for everything - although I think the main problem will be drunken hooligans."
The police in Hamburg carried out a large exercise on Wednesday dealing with the hooligan threat.
The fears over violence and disaster at the World Cup are fuelling an intense debate in Germany.
Interior Minister Wolfgang Schaueble has even suggested using the army for added security. But his predecessor, Otto Schily, has warned against a sense of hysteria.
"They're not trained for this kind of work and they're not equipped for it, so this should be done by the police," he said.
"We shouldn't create an atmosphere which suggests to our guests that we're getting ready for a civil war."
With helicopters overhead and soldiers bearing injured people away on stretchers, Saturday's exercise did look dramatic. But the authorities stressed they were not trying to frighten anyone - they were merely getting ready just in case.
Operation Triangle was the last major exercise before the World Cup. But smaller scale ones will take place in the three months before the tournament kicks off.