NANTES, France (AP) _ A huge chemical cloud from a fire in a fertilizer silo forced the evacuation Thursday of about 25,000 people and slightly injured 24, authorities said.

The cloud posed no danger of toxicity, but a portion of the adjacent Loire River would likely be polluted after technicians cooled down the silo to avoid risks of explosion, said Interior Minister Charles Pasqua.

Up to 1,000 police officers and gendarmes were mobilized to clear seven districts around this Atlantic port city at the mouth of the Loire. They were aided by helicopters and teams of chemists and technicians.

Pasqua said 24 people were stricken after exposure to the fumes and eight of them had been hospitalized for further tests.

The fumes emanated from a port silo where 850 tons of nitrate and ammonium, chemical components of fertilizer, continued to burn some 12 hours after the fire started.

The Environment Ministry said the cloud was not toxic, but the chemicals contained in it could irritate. ''There is no reason to panic,'' a spokesman said.

Officials speculated the fire, which began about 9:30 a.m., could have been sparked by a short circuit after an electric cable fell.

The fire was under control by 5 p.m., but continued to burn slowly Thursday night, officials said.