MANILA, SEPT. 18 -- Negotiations between the United States and the Philippines on the future of U.S. military bases here formally opened today, but potential snags promptly arose over Philippine demands for a vastly expanded agenda and a U.S. suggestion to leave a key role to future voters.

In an opening statement, U.S. special negotiator Richard Armitage acknowledged publicly for the first time that the days of a large American military presence here "are coming to an end." But he appealed for an unspecified "appropriate time" in which to make the changes.

U.S. officials have said the United States wants the Philippines to agree to a phase-out period of up to 10 years, leaving open the possibility of continued U.S. military access to the facilities after that on a commercial basis.

At issue in the talks are Clark Air Base, Subic Bay Naval Base and four smaller installations, which serve to anchor a U.S. military presence in Southeast Asia and help project American naval and air power into the Indian Ocean and Persian Gulf.

The bases negotiations, a highly divisive issue in Philippine politics, have sparked violent demonstrations by leftist groups this week and heightened U.S. fears of assassinations by Communist rebels. About 1,000 leftist workers and students today tried to march to the site of the talks at Manila's heavily guarded Central Bank building, but dispersed peacefully when riot police blocked their route.

Monday night, guerrillas of the Communist New People's Army tried to blow up a Voice of America relay tower at Capas, about 60 miles north of Manila where Camp O'Donnell, one of the remaining U.S. bases is located, to protest "domination by U.S. imperialism" in the country. A rebel communique blamed the U.S. presence in the Philippines for its "lack of development" and "current economic crisis."

A U.S. Embassy spokesman said the blast did not affect VOA broadcasts, causing only minor damage and no casualties.

After arguing in his speech today that a "sudden liquidation" of the bases would be "destructive" to both the United States and the Philippines, Armitage appealed to Philippine negotiators to "let the voters of the 21st century decide whether or not they believe a continued U.S. presence will be helpful."

Hours later, the Philippine side issued a statement welcoming some aspects of the U.S. position, but noting pointedly that "we take exception" to leaving the fate of the bases to future voters. "This matter is not to be decided in the 21st century," the statement said. "The time for decision is now."

The statement by the Philippine panel's spokesman, Tourism Undersecretary Rafael M. Alunan III, also demanded that the agenda be expanded far beyond security and defense matters.

"At these talks, we shall discuss issues of trade, aid, debt management, health, environment, education, veterans' rights, science and technology and other vital concerns," the statement said. "We expect the U.S. to present their proposals tomorrow regarding these matters."

A U.S. spokesman, Stanley Schrager, said the American side was "willing to discuss all these matters at a reasonable time, in a reasonable place and with a reasonable cast," but was not prepared to go into them in detail now.

He said Armitage's reference to future voters was not meant as a specific proposal, but reflected Washington's desire for "some kind of continued U.S. access to military facilities in the Philippines well into the 21st century."

In an opening speech for the Philippine side, Foreign Secretary Raul Manglapus insisted that "there must be more to U.S.-Philippine relations than marching to the same military beat." He scorned the idea that Manila's need for bases-related U.S. aid and expenditures -- now running at about $1 billion a year -- would dictate the outcome of the talks.

"We should not allow the dollar sign to be stamped as the indelible logo of these negotiations," he said.

The Philippines' 1987 constitution calls for the removal of all foreign military installations and troops from the country when the current bases agreement with the United States expires Sept. 16, 1991, unless an extension is ratified by the Philippine Senate in a treaty or approved in a national referendum.

In a televised address Monday, President Corazon Aquino called for a negotiated "orderly withdrawal" of U.S. forces. Her statement appeared to back a phase-out of the bases rather than the position of leftist and nationalist anti-bases activists, who demand the immediate removal of the installations once the current agreement expires.