PHILADELPHIA, SEPT. 12 -- It's easy not to envy the folks at We The People 200 Inc.

As organizers of the yearlong celebration commemorating the bicentennial of the Constitution, they had to generate enthusiasm for an old, yellowed document that most people regard somewhat hazily, if at all, without succumbing to the crass commercialization that has marked other national birthday parties.

One by one, they explored the possibilities and narrowed the list. There would be fireworks and a 30-float, 20,000-person parade through a two-mile stretch of this historic city, but no Benjamin Franklin look-alike contests or official Constitution rental cars. The cast of a nationally televised entertainment special would have to share equal billing with the Magna Carta and participants in forums on racism and the Equal Rights Amendment.

While federal officials and foreign diplomats lunched at Constitution Pavilion and aboard a luxury cruise ship, tourists and residents would be treated to a three-day, open-air picnic along the Delaware River. Philadelphia's two Democratic mayoral candidates would be asked to keep their campaigning far from Independence Hall, where former chief justice Warren E. Burger would be ringing a replica of the Liberty Bell and a band comprising college musicians from all 50 states would be performing.

Part history lesson, part hoopla, Constitution Week officially got off to a slow and unceremonious start here on Friday, less than three months after Vice President Bush joined 1 million people to celebrate the convening of the Constitutional Convention. The real success of the $21.5 million event, however, won't be known until its climax on Thursday, the aniversary of the date when 55 delegates from 12 of the 13 original states put pen to paper and created a government.

"The aim has always been to make the entire country aware of the Constitution," said Fred Stein, executive director of We The People 200. "Sometimes we have to do that with fireworks and festivals. Hopefully, we've done it with restraint and good taste."

We The People 200 has purposely avoided making crowd estimates for the coming week but is assuming the celebration will be smaller and saner than the Statue of Liberty's centennial last year. The last best guess by Stein and others is that 500,000 people will turn out for Thursday's parade and picnic, compared to the millions who went to Liberty Weekend in New York.

Although attendance at Independence Hall National Park, where both the Declaration of Independence and the Constitution were drafted, has increased by more than a third this summer, visitors to the two museum exhibits honoring the Constitution this weekend had the luxury of short lines. The few souvenir vendors staked out around the glass-shrouded Liberty Bell said business has been terrible since Labor Day.

"I don't think it has been promoted the way it deserves," said Linda McDaniel, who decided to see some of the exhibits on her way to the Penn State-Alabama football game.

Even so, all of the city's 8,000 downtown hotel rooms are booked and few of the 5,000 accommodations remain in the suburbs for Thursday, which has been officially designated Constitution Day. Beginning at 9 a.m., grand marshals Walter Cronkite and Coretta Scott King will lead off the main event, the We The People 200 Parade.

Produced by Radio City Music Hall Productions, the televised parade is arranged in three parts. The first part, complete with a replica of Maryland's Ship of State, The Federalist, no less than six fife-and-drum corps and 39 descendents of the Constitution's signers, is a recreation of the pageant that followed the Constitution's ratification in 1788 and its centennial in 1887.

The second segment is a visual representation of the five major themes in the Preamble to the Constitution. During the "Provide for the Common Defense" section, for example, floats and marching bands will honor the U.S. military, while "Promote the General Welfare" includes Robocop, a float depicting sports and recreation activities, and a 50-foot-high tribute to the construction industry.

A 13,000-member procession of ethnic, labor and civic groups marks the parade's final segment. With 5,000 participants, the AFL-CIO is expected to be the largest group represented.

President Reagan is supposed to give a short speech and release 1,500 white doves, but won't be staying for dinner. Also scheduled are the Mormon Tabernacle Choir, the 16 bankers and lawyers who make up the Synchronized Briefcase Drill Team and more than 2,000 journalists from around the world. Lech Walesa, the Polish Solidarity leader, put an end to a rumor he was coming, but said he would have liked to.

Organizers are still uncertain whether the throngs of visitors will ever materialize. Although Mayor W. Wilson Goode (D) had declared Thursday a city holiday, the federal government refused to follow suit and many of the city's largest businesses have already told the Chamber of Commerce that they do not plan to close. The plans of several news organizations whose resources are tied up with the visit of Pope John Paul II are unclear. With the confirmation hearings on Supreme Court nominee Robert H. Bork about to begin in Washington, only two congressional representatives have so far agreed to ride in antique cars that are supposed to follow a float celebrating the three branches of government.

But make no mistake about it: this is Philadelphia's show. Washington may have the annual Fourth of July fireworks extravaganza on the Mall. The tall ships that graced New York harbor for the nation's Bicentennial in 1976 and the Lady Liberty fest may belong to Manhattan. But only in the City of Brotherly Love will 200 new Americans celebrate the Constitution's bicentennial by taking citizenship oaths at a baseball stadium before a crowd of flag-waving Phillies fans.

Many events this week are of the home-grown variety. The Mother Bethel African Methodist Episcopal Church, the nation's oldest continuously owned black church and first stop on a weekend black history tour, is displaying its 1794 charter. Congregation Mikveh Israel, the city's oldest synagogue, is holding a celebration. And residents will get their first glimpse Monday of the restored statue of founder William Penn on City Hall.

Several residents interviewed said they are glad their city is having another chance to show its stuff. They noted that its 1976 Bicentennial celebration was marred when then-mayor Frank L. Rizzo tried to call in 15,000 National Guard troops because of a protest. Last year, several Statue of Liberty events were called off due to a strike by municipal workers.

"I think people are beginning to get into the spirit," said resident Gary Aubry. "One thing Philly does love is a parade."