Wednesday night's curfew and massive police presence in Harvard Square marked the beginning of a crackdown on street people in the area.

The Square's merchants, badly shaken by three large trashing episodes since the spring, met Wednesday and asked the city government to take strong action.

"We asked police to start redressing the balance to some extent." Alexander Zavelle, manager of the Coop and spokesman for the merchants, said yesterday.

"It amounts to saying, 'Kids, if you don't have something to do here, you'd better leave Cambridge,'" Zavelle said. "I don't enjoy saying that," he added.

Hard Hit

But the merchants feel they have little choice. Some have been hit very hard; Saks Fifth Avenue was closed for a week after the most recent widespread trashing and has suffered $40,000 damage in the three incidents.

"We're talking about the people who just stay," Zavelle said, and not all young people visiting Cambridge, "There is a small but definite group who just want to stir up trouble."

That "small but definite group" amounted to about 25 young people on Wednesday. A crowd of 100 to 150 youths had gathered for a planned "block party" on the Common at 8:30 p.m. As usual, marijuana was smoked, fires lit in trash barrels, and political slogans chanted periodically.

Many in the group said they expected a police attack in the Common at 9 p.m.-the hour set as curfew there by the City Council last week. But no police came.

Meanwhile. Cambridge police had moved over 200 men to the outskirts of the Square and at 9:30 p.m. City Manager Joseph Corcoran ordered an 11 p.m. citywide curfew. Police broadcast the curfew notice by amplifiers from their cruisers. About two dozen helmeted police were stationed at intersections throughout the Square.

Several times throughout the night, a handful of youths in the Common picked up rocks and tried to prod the crowd toward the Square. "Come on, I need a shirt," one shouted. But the crowd stayed put.

Barricade

Finally, shortly after 11, the group went into Massachusetts Ave., stoned passing cars and emergency equipment, and began ripping down a wooden fence to form a barricade.

Most of the 100 people present remained bystanders or tried to persuade the others not to trash or loot. About 25 seemed to want a confrontation.

Shortly after 11:30, about 60 police in riot gear began moving toward the group in the street, firing several volleys of tear gas. "They came out ofnowhere." one youth said later. The kids ran across the Common, trashed and looted two stores to the north, and burned a car before dispersing.

The police advanced slowly, giving the youths a chance to escape.

About the time of the sweep into the Common area. dozens of dark-suited Metropolitan District Commission police moved into positions on Mass Ave. and Mt. Auburn St., carrying shields and shotguns. The handful of people who had remained in the Square after the curfew left shortly afterward. Traffic was sent to side streets.