A cadre of trained Harvard security personnel took up strategic positions at entrances to the Yard, posting barricades and keeping a watchful eye. Their mission: to prevent illegal infiltration by violators of a long-standing regulation barring bicycling in the historic green.

Saul L. Chafin, chief of University police, said yesterday the new enforcement policy is needed to safeguard pedestrians, since "the incidence of bicycle riding has increased so much."

He did not say how much it is costing the University to pay the six additional guards that, he said, will be posted daily until further notice.

Although guards go off duty at 6 p.m., Chafin said he hopes the new policy will reduce the chances of a "catastrophe" like three bicycle-related deaths in New York City this summer. Notices were also tacked up to urge riders to cooperate.

Initial reaction among cyclists asked to walk their bicycles through the Yard was favorable. "Most of them get off and understand, although there's always the few that don't," Frank Naugler, one of six guards posted yesterday, said.

Daniel Steiner '54, general counsel to the University, said yesterday the new policy has "aesthetic" as well as safety considerations.