An attempted daylight robbery of a Brinks armored car delivering cash to a Harvard Square bank was foiled yesterday when its guard opened fire, wounding at least two suspects and sending a hail of bullets sailing across a crowded Harvard Square.

The two wounded suspects were heavily guarded last night at Beth Israel Hospital, one in critical condition in the operating room.

FBI agents as well as Cambridge, Harvard and state police are continuing a hunt for a third suspect, who escaped. Police consider the suspect, who fled through Harvard Square buildings, to be "armed and dangerous," Cambridge Police Detective Frank T. Pasquarello said yesterday.

Cambridge police and several witnesses said the violence started when the would-be robbers struck and threatened the guard as he wheeled in sacks of cash on a dolly into the vestibule of the Bank of Boston, in the center of Harvard Square.

"Don't make a move or I'll kill you," one of the suspects told the guard, according to Pasquarello.

The suspects had been waiting in the bank's vestibule for the guard, who was bringing in cash.

One of the suspects threatened the guard with a gun, took one of the cloth sacks of cash and then fled, according to witnesses. The guard ran after the two men and opened fire, hitting one suspect in the back and another in the head and arm, police said.

Injured, the two suspects attempted to climb into a getaway car, but their flight was prevented when they crashed into a Cutlass Supreme a few feet ahead on Dunster Street, near the corner of Mass. Ave. Cambridge police officers and ambulances arrived seconds later, taking the suspects, both bleeding, into custody.

At least six rounds of ammunition--one from the suspects and at least five from the guard--were fired in the exchange, Pasquarello said.

The guard, whom police refused to identify, was not wounded.

The third suspect fled down Dunster Street, and for several hours was believed to be hiding in the Dunster Street garage or in University Health Services (UHS).

For three hours, Cambridge police thoroughly searched all UHS offices with trained dogs, but announced their failure to find him around 5 p.m. The suspect, who remains at large, is described as a white male in his 20s.

Lunchtime crowds of Cantabrigians and Harvard students ran in terror as the shots were fired, hiding. between cars, under tables and behind theconcrete benches outside the Dunster St. Au BonPain. No bystander injuries were reported.

Traffic halted as the bullets careened acrossthe Square. Some motorists tried to drive inreverse down the busy, one-way avenue in anattempt to get out of the line of fire.

Pasquarello said police recovered the stolensack of cash and several firearms from thewould-be getaway car, a four-door maroon 1991Buick Century. Ballistic tests identified theweapons as a .357 magnum, a 9-millimeter pistol,and a Tech 9 semiautomatic weapon. The weaponswere fully loaded.

"It's unbelievable the amount of firepower theyhad," said detective Pasquarello. "It'sunfortunate that anyone had to get shot, but theybrought it on themselves."