In the third armed robbery of an undergraduate in less than two weeks, a student was attacked and robbed at knife-point Saturday afternoon just outside Lamont Library, the Harvard Police said yesterday in a public advisory.

The student, who police declined to identify but said lives off-campus, was attacked from behind by two assailants as he walked along Quincy Street near Lamont Gate around 3:50 p.m.

According to police, the assailants struck the victim on the head, knocked him to the ground, pointed a large knife at his torso and demanded "the contents of his wallet."

The victim was apparently struck by the fist of one of the attackers, not by a weapon, said Peggy McNamara, spokesperson for the Harvard Police.

After the attack, the assailants fled down Quincy Street toward Broadway Street, the advisory said. They escaped with $90 in cash and several credit cards, according to McNamara.

The police described the knife-bearing suspect as a white male, 20 to 25 years old, 5'10" to 6' tall, with brown bushy or wind-blown hair.

He was wearing a white T-shirt, dark blue jeans and white tennis shoes, according to the advisory.

The second suspect is described as a Latino male, also 20 to 25 years old, 5'10", with close-cut, well-groomed black hair, wearing a "dirty looking" blue shirt with collar.

For unknown reasons, McNamara said, the victim did not report the attack to the police until "some time" after it occurred.

The victim sustained minor injuries from the assault but received no medical treatment, McNamara said.

The police have no knowledge of any bystanders or witnesses to the crime, she added.

Saturday's attack occurred three days after two men, identifying themselves as undergraduates, robbed a first-year through a window in his Wigglesworth Hall room.

Both assaults took place in the same vicinity, and each involved a pair of assailants described as being in their early 20s, one of whom was white and one of whom was Hispanic or Latino, according to the police.

In both cases, the assailants demanded money at knife-point, according to police reports.

The detectives investigating the incidents were not available last night for comment.