WASHINGTON (AP) _ A radical group holds hostages in the defense secretary's office. Toxic chemical agents are accidentally released. Police and emergency vehicles rush to the Pentagon.

The simulated crisis is part of an exercise planned Saturday, the Defense Department announced Thursday in a note to news media who will be barred from the building during the emergency training activity.

The exercise, called ``Cloudy Office,'' is to last 10 hours and is expected to involve up to 700 volunteers and personnel from Defense Secretary William Cohen's office, the FBI, local emergency services, medical personnel and a special unit of the Defense Protective Service, the Pentagon's civilian police force.

``We've advised volunteers not to wear their nicest clothes and to wear swim suits underneath,'' since some will have to be stripped to wash off simulated chemicals, said Glenn Flood, a Pentagon spokesman. He said Cohen will not be among those held in his office.

Flood said all those involved will know it is an exercise and reminders will be broadcast on police radio bands to alert anyone listening who might think the crisis is real.

The exercise aims to test the response of civilian emergency units to hostage-taking and the setting up of barricades, complicated by the use of weapons of mass destruction in the nation's capital.

The simulated radical group is not identified, but it is supposed to be demanding U.S. intervention in a country threatening to use weapons of mass destruction. No reference is made to a specific country, and the scenario was planned before India and Pakistan entered into a nuclear arms race, Flood said.

``During a period of negotiation, there will be an accidental release of a simulated chemical agent that will require a response to a mass casualty incident involving contamination of personnel,'' said the memo to news media.

Reporters and photographers are to be kept in specific areas outside the Pentagon, much as they would be in a real emergency, Flood said.