The afternoon lull in Harvard Square was disturbed yesterday as nearly 200 chanting, placard-carrying marchers filed down Mass. Ave., protesting U.S. involvement in Vietnam.

The march, designed to "reach people with the facts about Vietnam," was sponsored by the Ad Hoc Committee on Vietnam, a group of Harvard and Radcliffe students, which includes members of the May Second Movement and the Students for a Democratic Society. Their destination was the Central Square Post Office, where the students planned to mail postcards of protest to President Johnson.

Cambridge Police sent 40 patrolmen and 40 plainclothesmen to accompany the marchers who gathered on the Cambridge Common. At first, the policemen refused to allow all but ten of the marchers to walk through the Square, insisting that they would cause confusion and tie up traffic.

The ten returned to report that Harvard Square was nearly empty, and the officers relented, permitting everyone to march single file through the Square and down to the Post Office.

The students carried various placards, chanted "Bring the troops home," and distributed mimeographed material, as police cars cruised beside them.

At the Post Office, each demonstrator mailed his card, and the majority returned to the Common to hear a short speech on the war's history. As the demonstrators disbanded one heckler who seemed to miss the point, declared "If you wanna do something for your country, why don't you all march down to recruiting headquarters and join the service?"

A Vigil for Peace in Vietnam, sponsored by the Turn Toward Peace, will take place today from 4 to 5:30 p.m. at the Boston Common, on the corner of Park and Tremont Sts.