PHILADELPHIA, SEPT. 15 -- In 1976, about 30,000 demonstrators, armed with babies and banners, took to the streets of this city to protest what they saw as "a flawed and incomplete American Revolution."

While city leaders and President Gerald R. Ford celebrated the nation's bicentennial at Independence Hall with a grand parade and speeches, the protesters, including Native Americans and Puerto Rican nationalists, gathered at a park four miles away to spotlight the effects of racism, sexism, military spending and social services cutbacks.

Eleven years later, many of the issues are the same. But what is different as organizers put the finishing touches on Thursday's birthday party for the Constitution is that protest will largely be a part of the official proceedings.

Fred Stein, executive director of We the People 200 Inc., the nonprofit organization charged with coordinating the festivities, said that he has tried to include representatives of ethnic and civic groups whose views were absent during the Constitutional Convention of 1787.

Today, representatives from the National Park Service and the U.S. attorney's office met with members of the Pledge of Resistance to go over the group's plans for a Thursday afternoon rally protesting continued aid to the Nicaraguan contras. Mayor W. Wilson Goode also issued a letter to the group guaranteeing that they would not be harassed by Philadelphia police.

In 1976, in contrast, Mayor Frank L. Rizzo asked the Justice Department to send 15,000 federal troops to the July 4 celebration amid rumors that protest groups would be engaging in terrorist activities. The request was denied.

"I knew going in that if we were just going to celebrate what happened 200 years ago, I would be excluding more than half of the American people from the celebration," Stein said.

A look at the calendar of events published by We the People 200 reflects Stein's effort. Among the programs it lists are last weekend's Hispanic Heritage festival, tonight's discussion on "Women and the Constitution" and a forthcoming forum, featuring civil rights activist Julian Bond and TransAfrica director Randall Robinson, on the relationship between the Constitution and American blacks.

Peter McDonald, chairman of the 200,000-member Navajo Nation, will be an honorary grand marshal of the three-hour parade that will wind through the center of Philadelphia Thursday morning. On Friday, Goode is expected to proclaim Sept. 21-27 Puerto Rican Week.

But the main vehicle for potentially dissident voices is the Parade of America's People. Scheduled as the third and final segment of the massive parade that will feature a speech by President Reagan, America's People includes representatives from such divergent groups as Toward a Gayer Bicentennial Committee, Americans for a Pro-Life Amendment, and the Pledge of Resistance.

Of the 13,000 people expected to march in that part of the parade, the vast majority are not members of so-called protest groups. Participants include 12 employes of Little Caesar's Pizza, 83 Girl Scouts and 200 disabled veterans. No group that applied was turned away. "What we have tried to do to a positive effect is mainstream the protest groups so they can't protest the parade or the picnic," Stein said.

At least one organization, however, has found fault with the setup.

The Bicentennial Citizen's Center, an information service set up here under the direction of consumer activist Ralph Nader, has complained that We the People 200 has attempted to hide protest groups by placing them in the final part of the parade.

Jason Adkins, the center's director, charged that We the People 200's decision was "calculated tokenism for protesters," because the segment was scheduled to pass Independence Hall after the president's speech and the nationally televised portion of the parade.

The center quarreled with We the People 200 after its request to sponsor a placard-carrying cadre behind a First Amendment float was denied. Stein, in explaining the denial, said the center had made its request too late to be featured with the float.