Police ordered additional security at some schools again today after racial tensions flared for the third straight day.

Three cars were overturned, windshields on a police cruiser and motorcycle were smashed, and a bus carrying black students was stoned during a march on City Hall by 150 white students protesting inadequate security at East Boston High School.

The flare-up, in which four students were arrested, was triggered by a scruffle at the school Wednesday where an 18-year-old white student was stabbed by a 16-year-old black.

Community leaders say escalating tensions, sparked by the Sept. 28 shooting of a black high school football player, stem from long-festering frustrations over jobs, housing and public safety as well as court-ordered school desegregation.

The U.S. attorney and the FBI were called in Wednesday to investigate "intimidation and violence" against black students whose buses have been stoned.

Two adults were arrested in front of City Hall Wednesday night when they defied a police order to disperse.

South Boston High School, the main forum for the fiery debate over "forced busing" several years ago, was calm today, with only about 100 juniors and seniors attending. Sophomores are due back Friday and freshmen on Monday.

The increasingly hostile racial atmosphere has prompted a series of meetings between Mayor Kevin White and community leaders. Cardinal Humberto Medeiros has called a second meeting Friday for community religious leaders.