MANILA, SEPT. 18 (TUESDAY) -- The head of a U.S. negotiating team publicly acknowledged today that the long American military presence in the Philippines is "coming to an end" and called for both sides engaging in talks on the future of U.S. military bases here to agree on an "appropriate" transition period.

The statement by U.S. special negotiator Richard Armitage came one day after Philippine riot police battled anti-American protesters near the U.S. Embassy, and President Corazon Aquino called for the "orderly withdrawal" of American forces from the country.

At least 20 persons were injured, including a dozen policemen and two Filipino reporters, in two separate clashes near the U.S. Embassy Monday as the American negotiating team arrived in Manila to discuss the fate of Clark Air Base, Subic Bay Naval Base and four smaller facilities.

At least 16 students and workers were arrested in the incidents, in which police used tear gas and truncheons to disperse more than 1,000 protesters, some of them armed with small bombs made of gunpowder and nails.

As the talks opened today under heavy security at Manila's Central Bank building, Armitage said in a prepared statement: "It is clear to me that the days of a very large presence of U.S. sailors and airmen in the Philippines are coming to an end. What remains for us to determine is the rate at which this presence will be reduced."

He said it was in both countries' interest to agree on an "appropriate" transition period.

In an opening statement for the Philippine side, Foreign Secretary Raul Manglapus said the principle of sovereignty would guide his delegation, adding that "we should not allow the dollar sign to be stamped as the indelible logo of these negotiations."

The second consecutive day of violence in front of the U.S. Embassy Monday contributed to an atmosphere of tension surrounding the bases talks. Citing intelligence reports that Communist rebels may try to assassinate Americans to protest the meeting, U.S. military commanders have suspended all local liberty and "nonessential" off-base travel by servicemen.

Ten Americans have been killed by suspected Communist guerrilla assassination squads since 1987, including two airmen gunned down outside Clark on the eve of exploratory talks between the United States and the Philippines in May.

In a televised speech Monday night, Aquino made it clear that the days of the 92-year-old U.S. military presence in the Philippines are numbered, although she did not set a date for U.S. withdrawal.

"It is now necessary for our government to work with the United States for arrangements regarding the orderly withdrawal of their forces from our country," Aquino said. She said this position was in keeping with a provision in the Philippines' 1987 constitution barring any extension of the current U.S.-Philippine military bases agreement, which expires in September 1991, unless the accord is ratified as a treaty by the Philippine Senate.

The United States will propose a pullout over a period of about 10 years, American officials have said.

The collapse of communism and the declining Soviet threat have led U.S. officials to conclude that the bases in the Philippines -- once considered vital to the projection of American military might in the Pacific -- are no longer needed.

The U.S. position of not seeking a renewal of the bases agreement appears to have met with a generally favorable reaction here and defused some of the traditional criticism by anti-bases activists.

The public debate in the Philippines has now shifted largely from whether the bases should be removed to how long the transition period should be. Government sources said Philippine negotiators are expected to seek a phase-out period of less than 10 years.

A consensus appears to have emerged in recent weeks that a sudden U.S. pullout would hurt the Philippines as much as the United States. Even some long-time opponents of the bases now recognize that, given the country's current economic difficulties, the Philippines can ill afford to lose the 80,000 jobs that the bases generate and the approximately $1 billion a year in U.S. aid and expenditures connected with them.

Arrayed against any continuation of the U.S. military presence, even during a phase-out period, are the left-wing Philippine nationalists, the Communist revolutionary movement and rightist military rebels.