PARIS (AP) -- The nightmare scenario goes like this: Paris police spot two suspects in a subway station. One smashes a container of deadly sarin gas on the platform as a train pulls in.
The imaginary attack is at the center of an anti-terrorism exercise running until dawn Thursday to test French security and rescue services' readiness for the real thing.
The ``assault'' targets the Invalides Metro and suburban train station on the Left Bank of Paris, close to Les Invalides, a gold-domed former hospital that houses Napoleon Bonaparte's tomb.
About 500 police and firefighters wearing chemical suits will evacuate and secure the area, set up special decontamination tents for ``victims'' in a park next to the station and work to remove the threat.
The simulation illustrated the extent to which the French consider themselves a potential terrorist target. In the past two years, police have dismantled alleged terror cells and uncovered potential attack plots.
Interior Minister Nicolas Sarkozy said the simulation came out of a request that law enforcement had made to President Jacques Chirac.
``This exercise is a way of being ready, even if there is no specific threat,'' he told the National Assembly on Wednesday. Prime Minister Jean-Pierre Raffarin was also scheduled to attend.
The simulation will be followed by about 50 similar exercises around France next year, Sarkozy said.
Though the attack is make-believe, the threat of sarin gas is real.
The deadly nerve agent, developed by the Nazis, was used in a 1995 attack on the Tokyo subway by the doomsday cult, Aum Shinri Kyo, that killed 12 people and sickened thousands. The assault was one of the worst acts of urban terrorism until the Sept. 11, 2001, attacks in the United States.
Officials were not releasing many details on the exercise, saying they wanted to keep an element of surprise that would make it more realistic for fire, police, hospital and other officials who will participate.
Police said the simulation, which has been in the works for months, was designed to test plans prepared in the past two years for dealing with terror attacks with nuclear, chemical or biological weapons. The tests were prepared in reaction to the Sept. 11 attacks.
The exercise, the first of its kind in the French capital, comes less than a week after France reduced its terror alert status to the lowest level, signaling that the risk of attacks has receded. But officials and experts say France needs to remain on guard.
In December, French authorities dismantled what they said was a terror cell with ties to Chechen rebels and al-Qaida that planned bomb or toxic gas attacks in France and Russia.
``It's about time we had one of these exercises,'' said Francois Heisbourg, director of the Foundation for Strategic Research think-tank. While France has proved itself capable of coping with natural disasters, it has not had to deal with terror attacks with mass destruction weapons.
``It's one thing to be technically well prepared. ... It's another to be operating in an urban environment, in very confined spaces, having to worry about people and traffic,'' Heisbourg said in a phone interview.
Police in London staged a similar exercise in September, decontaminating scores of ``casualties'' from a simulated chemical weapons attack.