The benches outside of Holyoke Center, in the little park known as Forbes Plaza, are off limits to all after 6 p.m. That means hippies, and men in three-piece suits.

Two policemen, a University policeman and a Cambridge policeman, are on duty enforcing the restriction. Harvard, which owns the Forbes Plaza property, pays the policemen a combined total of $359 a week to patrol the area.

L. Gard Wiggins, administrative vice president of the University, said yesterday that the restriction was put into effect a month ago, "to keep what I would call undesirable persons out of the area."

"We got complaints from local residents and from me too," Wiggins said, "about people climbing in trees and things like that. Motorcycle gangs hung out there. They threatened pedestrians and motorists on Dunster Street."

Once the curfew was established, of course, it had to apply to all people--not just "undesirable" ones. "We can't discriminate," Wiggins admitted.

Wiggins said he does not expect the curfew to be necessary during the school year.

Police chief Robert Tonis said that the Harvard policeman is on duty from 6 p.m. to 2 a.m. and the Cambridge policeman is on duty from 8 p.m. to 1 a.m., but the curfew remains in effect until 8 a.m., although there is not always a policeman there to enforce it.