In a demonstration marked by violence only at the end, over 15,000 people gathered in Government Center in Boston yesterday in support of the Chicago Seven.

As the crowd was leaving after the rally about 5000 people streamed up Tremont St. towards the Park St. subway exit throwing rocks and smashing windows. Police rushed the group as it reached the subway, driving the demonstrators up Beacon Hill towards the State House.

Using clubs the police beat dozens to the ground and arrested at least 20. Four Boston City Hospital ambulances were called and their crews moved into Boston Common treating injured youths lying on the ground. Several policemen were also reportedly injured.

The demonstration began with a rally at 4 p. m. in the Common followed by a march to Government Center. A few windows were broken on the march up Tremont St., but police took no action and it was not until the end of the demonstration that the major damage was done.

Affinity Groups

In the Common demonstrators were instructed to form "affinity groups" of about eight and to march to Government Center with linked arms. As they moved down narrow Tremont St., they chanted and raised clenched fists at the people in the towering buildings which line the street. "Now I know how it felt to be in the charge of the Light Brigade," one demonstrator said.

As the crowd streamed into and filled the bizarre setting of Government Center, speakers exhorted them to "begin a revolution now." Viet Cong flags and banners lined the wall in front of City Hall.

Originally organizers of the demonstration had been denied a parade permit for the march. Late Wednesday, however, they reached an agreement with representatives of the City and changed the route of the parade from the intended march to the Justice Department Building in Post Office Square. The City then granted the parade permit.

March organizers said that their first parade permit request had been denied on the grounds that the protest would interfere with rush-hour traffic in the downtown shopping district.

Organizers of the march had predicted a crowd of between 5000 and 10,000 demonstrators, but police said that they were prepared to deal with 20,000. Mounted police and foot patrolmen were positioned along Tremont St. and in the Common, and paddy wagons were assembled behind the State House. No tear gas was used on the demon-strators, although hundreds had come with gas masks.

A crowd of about 300 remained in the Common even after the first police charge on the Park St. subway stop. Police again charged, wielding their three-foot clubs and driving squad cars to disperse the crowd. Chased into the streets, the Cemonstrators began breaking windows in the apartment buildings and private homes along Charles St. Eight plate-glass windows in store fronts at Beacon and Charles Sts. were broken by rocks thrown by the demon strator-many of them apparently of high school age.

In a press release last night the organizers of the demonstration said, "Our demonstration tock place as planned-the tone of the march was militant and spirited. The police, unprovoked, attacked the demonstrators as we attempted to disperse in the park area.

"All around this country people are rising up angry," the statement continued. "We will not be intimidated and we will not be defeated."