Undeterred by overcast skies and occasional showers, a festive crowd estimated at 500,000 filled the Washington Mall last night as the nation's capital celebrated Independence Day with concerts and an ear-splitting, 35-minute fireworks display.

It was a day of pomp and patriotism, beginning with the annual parade down Constitution Avenue that attracted about 15,000 people and culminating in a dazzling display of pyrotechnical wizardry high above the Washington Monument.

Late in the big fireworks show, however, four people in the crowd near the Washington Monument at 17th Street were injured seriously enough to be taken to hospitals when an incendiary device landed among them. U.S. Park Police said they believed that the device was part of the official display. In addition to those hospitalized, several people were treated on the scene, police said.

Despite the huge throng, yesterday's celebration had an almost small-town quality, as families bearing quilts and coolers turned the Mall into a vast picnic ground and Frisbees floated lazily through the air.

Authorities described the mood of the crowd as subdued and reported no major incidents, although there were about 30 arrests for disorderly conduct. Vendors complained that business was slow.

Although the crowd grew slowly during the afternoon, Metro spokeswoman Beverly Silverberg said a "floodtide" of passengers between 6 and 9 p.m. pushed total ridership to between 400,000 and 450,000.

Despite a brief bottleneck just after the end of the fireworks show, which forced the temporary closing of some subway platforms, Silverberg said the Capitol South station was empty by 11:15, and the last stragglers had cleared out of the Smithsonian station shortly after 11:30.

"It was a steady, heavy crowd of tired but nice people," Silverberg said. "You know . . . Fourth of July kind of people."

Elsewhere in the city, about 20,000 people turned out at the Capital Centre for a "Welcome Back" concert honoring Vietnam veterans and featuring such performers as Crosby, Stills and Nash, Kris Kristofferson and James Brown. (Story, Page B7)

"This gesture is long overdue," actor and emcee Jon Voight told the crowd, which included many Vietnam veterans and their families.

In other places around the region and the country, people passed the Fourth with barbecues, softball games and odes to the Constitution, the bicentennial of which is being celebrated this year.

Others indulged in a time-honored Independence Day tradition and headed to the beach.

Few places celebrate the nation's birthday with the vigor of its capital, a tradition that showed no sign of flagging yesterday.

More than a hundred marching bands performed in the Independence Day parade on Constitution Avenue, which featured an appearance by Mayor Marion Barry and a float consisting of a model of the Great Wall of China that was donated by China.

Among the marchers was a group of Vietnam veterans in battle fatigues, some in wheelchairs, whose ranks grew steadily as they exhorted others on the curbside to join them.

"C'mon, we had a nice little war. Come out and join us," one shouted.

Many in the crowd on the Mall arrived early to stake out the best spots from which to watch a free outdoor performance by the National Symphony Orchestra and the fireworks display, which began at 9:20.

"I've seen this 30 times and it's always a grand day," said Robert Houston of McLean.

Houston, 50, said he goes to the Mall for the Fourth of July because "you see a cross section of America. Look at all the different ethnic groups."

Sally Bagley, sitting on a quilt with friends near the base of the Washington Monument, pronounced herself "thrilled to be back" after four years overseas, most recently in Panama.

"I think every American should be sent overseas," she said. "That way they would learn the meaning of what America is all about."

Earlier in the day, at the Festival of American Folklife on the Mall, a man stood before a group of about 50 people demonstrating a lost culinary art: how to cook muskrat. "They're delicious if you do it right," he said.

Washington is a city of causes, and most of them seemed to be represented at yesterday's celebration.

Cynthia Johnson, circulating petitions on the Mall for a group called Women Strike for Peace, said she got the best response in front of the National Air and Space Museum. "We think this is a very patriotic thing to do, especially on this day," Johnson said.

In Lafayette Park near the White House, the Youth International Party, otherwise known as the Yippies, staged its annual Independence Day demonstration.

National Park Service police made four drug-related arrests at that event, according to a spokeswoman, but otherwise there were few problems. Lt. R.W. Reid of the U.S. Park Police described the crowd as well behaved, with a higher proportion of families than in recent years.

The relatively small afternoon crowd displeased the city's many vendors, who look on the Fourth of July as a potential windfall.

Haleem Siddiqi, a District resident trying to sell jewelry in front of the Washington Monument, said, " '85 was a good year, much better than this year. Things are very, very slow."

"Lousy," said Cheryl Fleet, who was selling watermelon at $1 a slice at a nearby stand. "People are bored here. There's nothing for them to do. It's pitiful. They should have a band or something. It doesn't have to be the Beach Boys. But we need something."

But there were few complaints from those in the crowd, most of whom seemed to get just what they had bargained for.

At the Sylvan Theater next to the monument, the crowd lapsed into respectful silence as Ethel Ennis performed a soulful rendition of "The Battle Hymn of the Republic." When the fireworks began 10 minutes later, many people stood with their hands over their hearts as the U.S. Navy Band played the national anthem.

In Ocean City, Md., yesterday, it seemed that the 250,000 visitors city officials had been expecting for the Fourth had arrived by midafternoon. The boardwalk was crowded, and hundreds of sunbathers stretched out on the sand, seemingly oblivious to cool breezes, cloudy skies and occasional rain.

The town marked the day with softball tournaments and a skateboard contest near the boardwalk, and two fireworks shows were planned for 9 p.m.

Even as early as 3 p.m., people were settling into prime viewing spots on the beach with coolers and beach towels. "Last time, we didn't leave the place where we were staying until after 5," said Carole Kivett of the District, who spent the holiday at Ocean City two years ago but could not force her way through the crowds in time to get a good view of the fireworks.

In the past, Paula Evans of the District spent July 4 in her back yard, but this year she went to Ocean City with her friend Myron Rowles, leaving the District at 4:30 a.m. Friday in a successful attempt to elude beach traffic. The water was cold, she pronounced, but the atmosphere was "relaxing. It's a change from barbecuing in the back yard."

Bill McAtee of Silver Spring had planted a large American flag next to his beach towel. "I thought I'd probably be the only one flying a flag on the beach," he said, and he was right. "I figured this is the one day when you can wave the flag without people making fun of you."

Staff writers Marc Lacey, Pierre Thomas and Tom Vesey contributed to this report.