CHATEAUGUAY, Quebec (AP) _ Residents angered by the month-old blockade of a commuter bridge by Mohawks attacked police with gasoline bombs, golf balls, asphalt and vegetables in a third night of violence in this Montreal suburb.

The clash Tuesday night came as the government began sending troops to the area, where Mohawks clashed with police and blocked highways in a dispute over what they say is ancestral land.

In Chateauguay, about 1,000 people milled near police barricades separating them from Mercier Bridge. About 100 young people in the crowd started throwing objects at police about 10 yards away.

Police fired a dozen canisters of tear gas were fired at the crowd as several ambulances rushed up to police lines, apparently to take injured officers away. The extent of injuries was not known, and no arrests were immediately reported.

Brig. Gen. Armand Roy, head of the Fifth Canadian Mechanized Brigade, said the troop deployment requested by Quebec Premier Robert Bourassa is ''an administrative maneuver, to be in a better position to respond to whatever is required.'' He did not elaborate.

The first 100 soldiers were sent Tuesday night. Gen. John de Chastelain, chief of defense staff, said in Ottawa that Roy had been given authority to use 4,400 soldiers. But Roy said he could not use that many because some of his brigade were already committed to a peace-keeping mission in Cyprus.

Bourassa asked last week that the troops be placed on standby in the dispute, which arose when the town of Oka, 18 miles west of Montreal, decided to expand a local golf course onto what the Mohawks claim is ancestral land.

A policeman was killed when a gun battle broke out during an early morning raid by police officer on July 11 to remove a barrier put up by the Mohawks to block access to the land. No responsibility has been established in the death.

Police and Mohawks have been in an armed standoff since then at Oka.

Joe Deom, a Mohawk spokesman in Oka, said the military move will have no effect on negotiations, which are about to start.

''We've already been notified by Ciaccia's office that the troops are only to be available if rioting starts again in Chateauguay,'' Deom said.

He was referring to the province's native affairs minister, John Ciaccia.

The army said the troops would move from Valcartier near Quebec City and set up in St-Benoit and Blainville near Oka, and in St-Remi and Farnham near Chateauguay.

Nightly clashes with police by non-Indian residents and demonstrations have been reported at the blocked Mercier Bridge into Montreal. Mohawks have closed off the bridge, inconveniencing commuters who bitterly protest police failure to dismantle the barricade.

Negotiations between Quebec authorities and Mohawks will begin as soon as 24 world observers take up positions at eight barricades, Gilles Tardif said.

He is vice president of the International Federation of Human Rights, which will help monitor negotiations.