About 25,000 demonstrators, brought together in a loose coalition of groups whose causes range from gay rights to Palestinian autonomy, marched on the Pentagon yesterday to protest US. military involvement in El Salvador and President Reagan's proposed cuts in comestic social programs.

Massing early yesterday morning in the Constitution Gardens, just north of the Lincoln Memorial, the orderly and youthful crowd formed a colorful river of jean-and-tee-shirt-clad humanity as they set out past the State Department and over Memorial Bridge to the Pentagon at about 1 p.m.

As they marched, carrying banners for their causes while licking ice-cream bars and taking pictures of each other with complicated camera gear, they chanted along to slogans broadcast from speakers mounted on the roof of a rented yellow Ryder truck:

Money for jobs, not for war, U.S. out of El Salvador.

Stop Atlanta murders, down with the Junta.

gay-straight, black-white, same struggle, same fight.

No cuts, no way, make the bankers pay.

Traffic was tied up for several hours as Lincoln Memorial Circle, parts of Constitution Avenue and 22nd and 23rd Streets, and one lane of Memorial Bridge were closed to allow the marchers to pass. U.S. Park Police and D.C. Police estimated the crowd at about 25,000.

The demonstration was reminiscent of the Vietnam antiwar rallies a decade ago. The crowd that gathered on the Pentagon's mall side on the breezy, sun-filled spring day included representatives of labor, religious, antidraft and antiwar groups, as well as homosexual, black, Indian and Hispanic rights groups.

But yesterday's demonstration differed markedly from those of the sixties that resulted in pitched battles between demonstrators and police. Yesterday's minions carried a few placards and repeated a few chants, but some also took time to eat picnic lunches, smoke marijuana, drink beer and work on their tans.

Only one arrest was made yesterday, when a youth threw a red substance on one of the Pentagon's columns, Federal Protection Service police reported. No serious injuries were reported.

By 5:30 p.m., the crowd began to disperse, riding Metro, walking across the Memorial Bridge and reloading into some of the 300-500 buses that brought them from New England, the Mideast and near South.

The well-orchestrated march, organized by the People's Anti-War Mobilization and the Ad Hoc May 3 Unity Committee, went off smoothly until late in the afternoon.

At about 3:15 p.m., a group of about 300 counter-demonstrators from the Collegiate Association for Research of Principles (CARP) -- a body associated with Rev. Moon's Unification Church which is calling for U.S. intervention El Salvador to rid it of Russian and Cuban communist influence -- marched toward the larger group from an area near the river entrance where they had been isolated by police.

Waving American flags and chanting "No More Cuba, No More Wr, Castro Out of El Salvador," the clean cut protestors from CARP formed a line along a service road near the portable toilets on the river side of the grassy Pentagon mall area. For several moments, the atmosphere the tense. The groups shouted vehemently back and forth as security volunteers from the larger demonstration lined up facing the CARP people, holding furled green banners outstretched at eye level like billy clubs.

But even as the CARP people continued to yell slogans, the demonstrators from the larger crowd stopped chanting, watched the CARP demonstrators and pointed their fingers and chuckled. The tension passed and soon the CARP demonstrators returned to thier former position.

Many of those interviewed yesterday -- from long-haired hippie holdouts with painted faces to L.L. Beanclad outdoorsmen to health-conscious joggers who had stopped by to witness the spectacle -- said they had come, not so much to protest U.S. intervention in El Salvador as to voice their disapproval of the state of the nation under Reagan and the state of the world in general.

In all, the demonstration took on a flea market atmosphere -- something for everyone. College students and Hispanic migrant farm workers, leftist feminists and government workers from the American Federation of Government Employees, Revolutionary Communist Party Workers and Navajos.

Why did they come?

Lloyd Helms, a retired lawyer, and his wife Nancy came to Washington from Ann Arbor, Mich. because, Nancy said: "We've been interested in peace for as long as we can remember. We hoped, when the Vietnam war was over, that the United States had learned some sense. But now they want to build up defense and go into a war posture. We felt it was time to come down and say something ourselves."

Carlin Meyer, soon-to-be-president of the New York City Chapter of the National Lawyers Guild, brought her group of a dozen because, she said: "We oppose the oppressive policies of the Reagan administration. We oppose the cutbacks for social services, especially legal aid . . . we consider ourselves fighters for the people world-wide."

Larry Smith, a 36-year-old Vietnam veteran who lost a leg while serving in the 1st Marine Division as a radioman came from Brooklyn with the Black Veterans for Social Justice. "We came back from the war," Smith said, "and we still faced the same bad things that we left. We fought and died for our country and yet there was still racism against us. Now there is still racism and they want our children to go fight a new war.

"We shouldn't be fighting the third world people. Out battle is here," Smith said.

As the afternoon waned, master of ceremonies Larry Holmes, his voice hoarse from an afternoon of chants and pleas for the hodge-podge collection to maintain future solidarity against U.S. intervention in El Salvador, made a new plea -- for money for the People's Anti-War Mobilization.

"Give $5 . . . or give $20 . . . or take out that checkbook and write us a check for as much as you can give. Reach down and give until it hurts to ensure that this won't be the last demonstration this spring. Let us not default."

And then, a a new chant:

What Do We Need?

MONEY!!!

When do We need it?

NOOOOOOOOOOW!!!!!