Police arrested a Medford man on assault and battery charges early Saturday morning after he allegedly participated in an assault of two Harvard undergraduates.

One of the students, Daniel J. Greaney '86-87, who is on leave this semester, was knocked unconcious and taken by ambulance to Cambridge City Hospital.

Two other students, Christopher J. Riley '86 of Lowell House and William Kaplan, originally a member of the class of '86, were with Greaney at about 1 a.m., waiting to be let in to Claverly Hall at 65 Mt. Auburn St.

According to Greaney and Riley, the fight began after Andrew R. Mann '86, whom they were visiting at Claverly, threw his keys down from his fifth floor room.

Greaney said yesterday that the keys landed a few feet from a group of four or five young men, who then threw a bottle at Mann's window and challenged the Harvard trio to fight.

Greaney, who spent 12 hours in the hospital, said he did not remember exactly how he had been beaten, but said "I must have been hit with something harder than a hand."

Harvard University Police arrested one of the alledged assailants, Allan Hamilton, at 1:18 a.m. in front of Claverly Hall. The University police then took him to Cambridge City Jail, since Harvard police headquarters does not have holding facilities.

Later police arrested two more of the alledged assailants, according to Harvard police records.

Police were responding to Kaplan, who was walking to the Cambridge City Hospital to visit Greaney at around 3 a.m., when he encountered two men whom he thought participated in the earlier fight, he said yesterday.

He ran to a Harvard police car, after the youths began throwing bottles at him, he said. The police then arrested the youths