Thousands of visitors who endured an Independence Day of soaking rains on the Mall were treated tonight to a dazzling, 20-minute display of fireworks, which lit up the low clouds over the Washington Monument despite weather that had discouraged turnout, canceled and relocated some musical events and halted the annual July 4 parade.

The evening's pyrotechnics followed the National Symphony Orchestra's free concert on the West Lawn of the U.S. Capitol, which also began as scheduled after occasional downpours throughout the morning and afternoon threatened to postpone the evening's festivities for the first time in 25 years.

Earlier in the day, approaching thunderstorms halted the two-hour Independence Day parade after less than a third of the participating bands, floats, balloons, military units, dancers and other groups had finished their trips down Constitution Avenue NW.

"We're disappointed, but the safety of the participants and the street crowd comes first," said John Best, producer of the parade, which included about 3,000 marchers and other participants. "We don't want to endanger anyone."

Attendance on the Mall appeared to be down this year, based on Metrorail ridership. As of 6 p.m., 178,000 passengers had used the Metrorail system -- about 30 percent fewer riders than by the same time last July 4, which fell on a Friday.

"It's more like a typical Sunday than like a typical Fourth of July," Metro spokesman Ray Feldmann said this afternoon.

Thunder, drenching rains and soggy ground were not enough to dampen the spirits of some visitors who arrived early to stakeout their spots.

"Hey, we're 98 percent water," said Robert Baylor, 38, of Arlington, who had come to the Mall this morning to help erect a 25-by-13 foot party tent festooned with flags and claim a prime position to watch the fireworks from directly under the Washington monument.

"This is the best seat in the house for the best fireworks show in the nation," Baylor said.

With rain pouring down early in the day, some visitors, including entire families, found refuge in rows of port-a-potties lining the Mall. "I wish I had a jacket," said Carlos Hurt, 46, of the District, as he sought shelter in one of the outhouses.

As the rain let up in the afternoon, hundreds of boats gathered on the Potomac near the Lincoln Memorial to take up position for tonight's big show.

Later in the day, another wave of rain arrived, along with gusting winds that turned Susan Walsh's green umbrella inside out. "This is the worst Fourth of July ever," said Walsh, 22, a recent college graduate from Eldersburg, Md., in town to see the fireworks with a group of friends from high school.

"It's horrible," said Amy Greco, 22, Wash's high school friend. "My feet are muddy, my shoes are wet, and I have pebbles between my toes."

Walsh, Greco and their friends hoped to watch the fireworks from the Mall, but gave up because of the rain. "I'd rather just watch on TV," said Chad Tassin, 22, also from Eldersburg. "I just want to be dry."

While the nationwide color-coded terror threat level remained at yellow, or "elevated," for the holiday weekend, security was tightened for the national birthday festivities -- as it was for the past two July 4 celebrations since the Sept. 11, 2001, terrorist attacks on the United States. In May, Attorney General John D. Ashcroft and FBI Director Robert S. Mueller III warned that the Independence Day celebration was among the major public events terrorists might plan to target this year.

The Mall was closed for a security sweep this morning, before 19 security checkpoints opened at 10 a.m. for anyone willing to brave occasional showers.

More than 20 local, state and federal agencies contributed to today's security. District police activated 14 surveillance cameras and joined the Park Police to man the checkpoints that provided access to the Mall.

Security also was on the minds of many tourists, including Mona Queoff, 38, of Madison, Wis., who came to the capital for the Independence Day holiday with her husband and children. "Seeing all of the security does make me think about it all. But that didn't stop me from coming because that is what they want," Queoff said, referring to terrorists.

The museums lining the Mall provided some shelter and distraction from today's dismal weather. The Smithsonian Folklife Festival also provided tent-covered performances and demonstrations by musicians, dancers, craftspeople, storytellers and artists focusing on this year's festival themes -- Latino culture, Haiti and mid-Atlantic maritime communities.

The fireworks followed an evening of music on the Mall. The National Symphony Orchestra's "Capitol Fourth" concert, hosted by actor Barry Bostwick, featured a medley of John Philip Sousa marches and Tchaikovsky's "1812 Overture," which was punctuated by cannons firing from the Reflecting Pool. Country-music star couple Vince Gill and Amy Grant, gospel singer Yolanda Adams, Robin Gibb of the Bee Gees and American Idol singer Clay Aiken joined the orchestra for performances. Erich Kunzel, America's premier pops conductor, also paid tribute to Ray Charles, who died last month.

A planned concert tribute to big-band era music featuring the U.S. Army Blues Jazz Ensemble and singer Marlena Shaw on the Washington Monument grounds was canceled because of weather. And a 6 p.m. outdoor performance of the U.S. Air Force Concert Band and Singing Sergeants was moved inside the National Air and Space Museum. Concert-goers set up beach chairs in the museum's lobby beneath a Boeing 247-D and Douglas DC-3 hanging from the ceiling.

The Metrorail system was scheduled to be open until midnight. The Smithsonian Metro station was closed for the day for security reasons, as it was for July 4 celebrations the past two years. The Federal Triangle, L'Enfant Plaza and Archives-Navy Memorial stations near the Mall were open.

Washington Post staff writer Amit R. Paley, Arielle Levin Becker and Hamil R. Harris contributed to this report.