Talks began Tuesday on the future of the last major U.S. military garrisons in Southeast Asia, and the chief U.S. negotiator said the outcome will affect U.S.-Philippine relations and regional security.

"The significant Soviet presence in the immediate area casts a shadow over expanding economic activity throughout the region," U.S. Ambassador Nicholas Platt, head of the 10-member American panel, said in opening remarks.The review of a 41-year-old agreement on U.S. bases in the Philippines comes amid a growing clamor by many Filipinos for phasing out the garrisons, which they see as a vestige of U.S. colonialism.

About 500 people marched Tuesday afternoon from the Philippine Senate to the U.S. Embassy chanting slogans demanding the bases be closed. Police stood guard in front of the embassy compound, but there were no incidents.

U.S. and Philippine officials described the opening session as cordial.

"Both sides expressed their desire for a successful review and pledged to work toward this end in a harmonious, business-like manner," the two panels said in a joint statement.

Foreign Secretary Raul Manglapus, chief Filipino negotiator, has called for substantial increases in the $180 million that Manila receives annually for use of the six installations.

Filipino officials say they will also press U.S. officials for details of nuclear weapons that may be stored there and for expanding the right to try American troops charged with criminal offenses.

The outcome of the talks, expected to last three months, will determine whether the Philippines allows the bases to remain in the nation after their lease expires in 1991.

Under a new Filipino Constitution, a lease extension must be approved by two-thirds of the 23-member Senate, where opposition to the bases is strong.

Platt has refused to say whether Washington will agree to a major increase in lease payments. Last month, Manglapus suggested U.S. payments of $1.2 billion for the bases.

Platt noted Tuesday the presence of American troops enables the Philippines to spend scarce resources on economic development instead of defense.

"An overnight shift in Soviet intentions can transform this presence into a serious economic and military threat to the Philippines and the region," Platt said.