TOKYO - TOKYO staged its biggest ever anti-terrorism exercise on Friday, in an attempt to prove its credentials for staging the 2016 Olympic Games, days after Barack Obama's US election victory boosted rival Chicago's hopes.

Sirens wailed as a thousand personnel drawn from the army, navy and coastguard, as well as police and rescue services, gave a carefully orchestrated demonstration of how they would deal with a nuclear 'dirty' bomb explosion at a conference centre.

Police abseiled from a helicopter and overpowered a group of men in balaclavas amid a cloud of smoke from fake gunfire.

'We wanted to prove that we are capable of dealing with whatever disaster should occur,' said Mr Rikiya Kondo, an army officer seconded to the Tokyo city government as director for crisis management, adding that he hoped the message would be heard around the world.

The dirty bomb scenario was chosen because it is considered to be relatively likely, given that radioactive material has been stolen from various countries, Mr Kondo said.

Japanese media have already begun speculating that Tokyo could lose out in the Olympic race to Chicago, the home town of US president-elect Barack Obama. But officials in the Japanese capital are still sounding a positive note after Tokyo gained the top score in a technical assessment in June.

Tokyo governor Shintaro Ishihara underscored the city's other strong points.

'When comparing the attractions of Tokyo with those of other major cities of the world, we can proudly say that a striking feature of Tokyo is its high level of security,' he said in a statement issued on Friday.

'Tokyo will become the safest city in the world,' he added.

The election of the host city for 2016 will take place in October next year at an International Olympic Committee (IOC) meeting in Copenhagen.

Tokyo previously hosted the Olympics in 1964. -- REUTERS