High levels of carbon monoxide in eight Upper West Side buildings forced the evacuation yesterday of 35 people, authorities said.

Firefighters responded at 3:30 p.m. to West 76th Street between Columbus Avenue and Central Park West, a tree-lined block of brownstones and townhouses.

Residents were allowed back in their homes at 8 p.m. No one was injured.

The source of the high levels of the potentially lethal gas wasn’t known last night.