President Ferdinand Marcos today warned his opponents not to underestimate the power of the Philippine armed forces as his government marked the country's Independence Day with the biggest show of military force here since the 1950s.

"It is time that we convey a message to the demagogues who preach violence without understanding what they are up against," Marcos said before reviewing a parade at the Luneta grandstand in a park by Manila Bay. The parade featured several thousand troops, including elite units in combat gear, about 50 armored personnel carriers and light tanks and a flyover by jet fighters, helicopters and World War II vintage T28 planes.

The government's Independence Day celebration coincided with the nomination of a presidential candidate by the country's largest moderate opposition political group and a demonstration by leftists who unsuccessfully tried to march to the U.S. Embassy here and shouted slogans supporting the Communist insurgency.

The New People's Army, the armed wing of the Communist Party of the Philippines, has an estimated strength of about 12,000 guerrillas and has been escalating its rural-based insurgency in recent months against Marcos' nearly 20-year-old government.

The United Nationalists Democratic Organization, a coalition of seven political parties, unanimously nominated its leader, former senator Salvador Laurel, for consideration by a future convention of all opposition groups as a candidate for president. He is the the first presidential nominee of a major opposition group. A convention will choose a single presidential candidate to oppose Marcos in the 1987 presidential election.

About a mile from the parade, in which about 7,000 military and civilian marchers participated, riot police blocked about 10,000 protesters who tried to march to the U.S. Embassy. The demonstrators eventually presented a resolution to the embassy demanding an end to U.S. "intervention" in the Philippines.