American recognition of Taiwan officially came to an end today quietly and with a bare minimum of ceremony.

There were no bands and no speeches. Sometime early this morning the U.S. Embassy became a nonembassy when a metal plaque was removed from the building ending 30 years of recognition of the Republic of China.

The closest thing to a ceremony occurred late yesterday afternoon when three Marine guards took down the embassy flag for the last time. The only audience was a group of about two dozen reporters.

About 30 Taiwanese police stood guard as the flag was lowered at 5:12 p.m. Children skate-boarded on a highway overpass above the embassy grounds. There was no hint of the violence that last week threatened a high-level U.S. delegation arriving for talks on future unofficial relations with Taiwan.

The U.S. officially switched its diplomatic recognition to the mainland today, and in Peking a number of official ceremonies were scheduled. But here the Americans were determined to play the changeover in a low key.

By chance, this is also the day that Taiwan celebrates as the 68th anniversary of the founding of the Republic of China, and the Taipei government officially took note of its persisting claim to be the government of all China. tWhen Taiwan's flag was raised in front of the presidential office a crowd of nearly 7,000 persons, mostly university students, gathered quietly. They carried banners expressing pleas for unity in the wake of the American Tecision to recognize Peking.

"Stand together," said one. "To save our nation we must depend on ourselves," said another.

Across the front of the presidential building, a large map of China embracing the mainland and Taiwan was outlined by electric lights. As the flag was raised at 6:30 a.m. a military band played the national anthem and 20 military police stood with fixed bayonets.

Rallies commemorating the founding of the republic and a speech by President Chiang Ching-kuo were scheduled for later in the day.

Although early this morning the guard at the U.S. Embassy was stiffened with a heavier police force and new riot gear, there were no signs of an encore of last week's passionate anti-Americanism.

The only public expression of anger against the United States came from the Foreign Ministry, which issued a statement denouncing the U.S. plan to abrogate its mutual security treaty with Taiwan in another year.

By acting without prior consultation, the Foreign Ministry declared, the United States had violated the spirit of the treaty.

U.S. officials have said here that most of the personnel at the embassy will remain at their posts until a new arrangement is worked out to maintain nongovernmental relations with Taipei. Just what that arrangement will be is still in doubt.American officials have said that the talks, begun amid violence last week, will be resumed sometime in the future.

It is assumed that the United States will establish a private association to carry on trade and cultural contacts with Taiwan and to perform some consular duties.

Taiwan has insisted that it wants to maintain direct government-to-government contacts with the United States, but American officials have ruled this out. Several other countries that already have broken relations with Taiwan maintain private, unofficial organizations here to continue trade and cultural relations. CAPTION: Picture, Reporters watch U.S. Embassy flag in Taiwan lowered for last time yesterday. UPI