Military police on Saturday fired tear gas at a crowd of demonstrators as officers detained a man and woman whom they said broke onto restricted U.S. Navy land.

The incidents occurred during the sixth day of military exercises on the outlying Puerto Rico island of Vieques.

Tear gas was used to disperse the crowd, which according to Navy spokesman Lt. Corey Barker, was throwing rocks at military personnel.

Witnesses denied the claim, saying the crowd began throwing canisters of the tear gas only after officers began firing.

Demonstrators routinely break onto Navy lands to thwart exercises on the firing range whose use by the Navy has raised anger that flared after off-target bombs killed a civilian guard in 1999.

On Friday night, military police fired tear gas at demonstrators who threw rocks at officers guarding the perimeter of the bombing range, Lt. Cmdr. Katherine Goode said Saturday.

Local police were called in to calm the crowd Friday. One protester, Jaime Collado, suffered a cut to the back of the head and received five stitches, said officer Cesar Gracia. Collado said there were no rocks thrown in the incident. Collado said he was injured by the aluminum tear gas canister used by the military police.

On Saturday, ship-to-shore shelling and air-to-ground exercises continued. The maneuvers could last for nearly three weeks. The Navy says protests have not stopped the bombing practices since they began Monday.

Opponents of the Navy exercises say they harm the environment and health of Vieques' 9,300 residents. The Navy denies that claim.

President Bush says the Navy will leave by 2003, and the Navy says it is looking for alternative sites.