A 10-square mile cloud of toxic chlorine gas leaked from an industrial plant and forced a 71/2-hour evacuation of up to 27,000 people east of Los Angeles on Saturday. At least seven people were taken to hospitals.

The gas cloud formed just after midnight outside a plant that makes chlorine tablets for swimming pools, authorities said. The chemical reaction that caused the cloud was brought under control about nine hours later.Residents were told they could return home after getting the OK from the Los Angeles Couunty Department of Health.

An estimated 550 gallons of a chemical used to make the chlorine tablets apparently became contaminated, setting off the reaction, said Sheriff's Deputy Van Mosley.

The air was permeated with the smell of bleach from the the wispy, smoky-gray gas that billowed from the Grow Group Inc. plant. Chlorine can be lethal in high concentrations but otherwise causes burning eyes and sore throats.

Firefighters and deputy sheriffs evacuated between 25,000 and 27,000 people in surrounding neighborhoods, about 10 miles east of downtown Los Angeles, Mosley said. Wind was light and variable.

Three people were in stable condition at Santa Marta Hospital and Clinic, said nursing supervisor Verna Brinson. Four others with respiratory complaints were treated at County-USC Medical Center and released, said Norka Manning, a hospital spokeswoman.

At the Pioneer High School gym, where hundreds of evacuees waited, many said they were notified by neighbors or knocked on doors themselves to rouse people.

Grace O'Brien of Montebello said she bundled up her 89-year-old mother when they left her home.

"I tried not to take deep breaths," she said. "I told my mother to roll up the car windows, and I made an illegal U-turn - I didn't think the deputies would stop me."

Like many other residents, O'Brien said she didn't smell anything or see the chemical cloud.

"Didn't bother me. I've been through so much . . . tornadoes, things like that," said O'Brien's mother, Marguerite Secrist, a former Texas resident. The area evacuated comprised several square miles in sections of East Los Angeles, Monterey Park, Montebello and Commerce, authorities said. Residents were sent to several high schools where the Red Cross had set up shelters.

The plant was closed at the time of the leak, said plant manager Kim Marti. "The sheriff's department woke us up," Marti said.