Environmentalists fledgling and veteran, as well as the just plain curious, jammed the Capitol grounds and the Mall yesterday for a giant celebration of music and exhortations to ecological activism that was the nation's flagship Earth Day rally.

"Welcome to the first day of the green future," Denis Hayes, chairman of Earth Day 1990, told a festive crowd that stretched from the Capitol grounds to the Washington Monument. "We hear the cry of the Earth, and we have come to heal her."

Capitol police estimated that 125,000 people were on the Capitol grounds at the height of the rally, but that estimate did not include 11 or 12 blocks of the Mall, which at times were packed with people. Organizers said at least 350,000 people attended.

The huge gathering on a balmy spring day had the flavor more of the annual Fourth of July birthday party than a political rally. Movie star Tom Cruise drew squeals when he appeared on stage, there were more picnics than placards, and waders splashed in the reflecting pool at the foot of the Capitol. The Earth Day T-shirt stand sold out.

Standing against the backdrop of a giant blue and green Earth Day sign, entertainers and politicians encouraged the crowd to pressure politicians and corporations to clean up their environmental acts and to launch a decade of activism on behalf of the planet.

Young volunteers in Earth Day T-shirts roamed the crowd distributing "green pledges," in which people promised to make environmental reforms in their daily lives.

The crowd was overwhelmingly white, despite organizers' hopes for a strong minority turnout, and many were college students who came in groups from their schools. Some young people camped out overnight to secure a choice spot near the stage on the Capitol steps.

Police reported only three arrests, two for disorderly conduct and one for attempted bicycle theft.

A male bicyclist, whose identity was not immediately available, was seriously injured in a collision with a truck at Constitution and New Jersey avenues NW.

About 200 people were treated for minor injuries at first aid tents, and five were sent to area hospitals for further treatment, according to Capitol police.

An estimated 3,600 American cities and towns held Earth Day events this year, marking the 20th anniversary of the day that energized the nation's environmental movement.

Earth Day organizers said 141 countries on all seven continents would celebrate the day, a contrast to 1970's U.S.-based event.

Although environmentalists hope that Earth Day will inspire new activism, some in the crowd wondered whether the celebrity-packed rally would become more than a media event.

"If the bands and the people they've heard of weren't here, I wonder if people would show up," said Patrick Meagher, 18, of the District. "I don't think they would. That's the whole problem with it. It's the trend."

But others differed on the day's impact: "I work in telephone sales," said Beverly Wolfe, 47, of Alexandria. "The customers don't always take them, but you've planted a seed. It's like this: If people don't know what to do, you'll remind them."

Several speakers at Washington's Earth Day rally mocked President Bush, who spent the day fishing in the Florida Keys and gave one of his "point of light" awards to the citizens group Reef Relief, which is working to save the coral reef flanking the Keys.

Randall Hayes, director of the Rainforest Action Network, blamed "a thousand points of selfishness and greed" for much of the environmental damage to the planet.

The day's speeches emphasized that Americans not only have to be more responsible by recycling, shopping better and wasting less, but they also should pressure their leaders to do more for the planet.

The issues addressed ranged from the destruction of the rain forests to the erosion of the planet's upper ozone layer. The day's theme was emblazoned in green on the black T-shirts of many of the speakers: "Who says we can't change the world?"

Earth Day observances began at dawn at the Lincoln Memorial, where students marched with candles and a reproduction of a globe. In Oak Ridge, Tenn., demonstrators in white death masks and black shrouds held a "requiem for the Earth" at the gates of a nuclear weapons plant. In Maine, 800 hikers carrying flashlights climbed Cadillac Mountain for a sunrise ceremony.

Later, hundreds of thousands of New Yorkers packed Central Park for an Earth Day concert, more than 100,000 came out for a rally in Chicago's Lincoln Park, and dozens of other cities held rallies and concerts.

In Gdansk, Poland, students bicycled through the Baltic port's Old Town to protest air pollution. Environmentalists in western Japan gathered and burned an estimated six tons of trash on a beach. In Chamonix, France, two ice sculptors carved a giant thermometer on the Mer de Glace glacier to protest global warming.

The celebrities at Washington's rally included actors Richard Gere, Woody Harrelson, Malcolm Jamal Warner and John Ritter; singers Olivia Newton-John and John Denver; rappers L.L. Cool J and KRS-1; pianist Bruce Hornsby; saxophonist Branford Marsalis; and music groups 10,000 Maniacs and Indigo Girls. There was even Sesame Street's Cookie Monster, described as "the original eco-freak," who told the rally: "Cookie Monster says, 'Never throw anything on the ground.' "

"This is a cloth diaper," said actor Kevin Bacon, holding one aloft. "They're easy to use. They have 1,001 uses and they don't pollute." Bacon and actress Kyra Sedgwick brought their 10-month-old son to the rally.

Some of the music was on an environmental theme, such as Denver's new song, "Raven's Child," based on his visit to Alaska's Prince William Sound, where the Exxon Valdez went aground last year and spilled nearly 11 million gallons of oil.

The crowd cheered when Denver announced that an American hostage had been freed in the Middle East.

The original Earth Day was criticized by some for falling on Lenin's birthday, but it is a mark of how mainstream the environmental movement has become that criticism this time is more muted.

"Join the struggle while you may -- the revolution is just a T-shirt away," sang Billy Bragg. He later sang the "Internationale," the Communist anthem, and told the crowd, "It's been a while since that was sung on the steps of the Capitol."

Former senator Gaylord Nelson, 74, told the gathering, "I don't want to come back here 20 years from now and have to tell your sons and daughters that you didn't do your duty."

For veteran environmentalists, one of the rally's most moving gestures came when Nelson, co-founder of the 1970 Earth Day, presented a sculpture made of branches of endangered trees to Rep. Morris K. Udall (D-Ariz.), for a lifetime of environmental work on behalf of Alaska lands, the Arizona wilderness and other causes.

Udall, who has Parkinson's disease, walked haltingly across the stage to accept the award. He grasped the lectern with both hands and urged his listeners to make a difference: "I am only one," he said, "but I can do something."

Many of those attending the rally heeded environmentalists' advice and took public transit. The result was that auto traffic was lighter than usual and the Metro system was jammed, with long lines outside the Smithsonian station. Spokeswoman Beverly Silverberg said ridership far exceeded Metro's anticipated 25,000 to 30,000.

Organizers promised to clean up and they did: Within an hour after the rally, virtually the entire area had been cleared of garbage, which was then sorted for recycling.