The 250,000 people who marked the 20th anniversary of the March on Washington arrived there on foot, train and bus--but not necessarily when officials thought they would.

Hundreds of buses arrived hours later than expected, and in droves rather than on a staggered schedule. But once demonstrators reached the Mall, officials said the massive celebration proceeded without major mishap.

The departure of the demonstrators was smoother. People began drifting away about 3 p.m., and the Mall was largely empty by 8 p.m. Metro trains and buses carried heavy loads of people back to staging areas at RFK Memorial Stadium and the Pentagon to catch buses for home.

While a few people were unable to find their buses, officials said most of the buses from out of town had left here by dark. City officials lined up sleeping space for people who had missed their buses home.

During the 11-hour celebration, more than 100 people had been taken to hospitals by midafternoon, most of them for heat-related ailments, while hundreds of others were treated at medical aid centers set up around the demonstration site.

But jugs filled with cool drinks, hats and more than 300 portable toilets helped most people in the vast crowd endure the 95-degree temperature.

Police said the crowd was remarkably orderly, with only 24 arrests reported by midafternoon. They said 15 of those were people charged with operating vending stands without licenses.

Nearly all of the D.C. police department's 3,600 officers were on duty at some time yesterday, Police Chief Maurice T. Turner said during a press conference last night.

About 50 members of the D.C. National Guard filled in for officers absent from outlying police districts.

Turner praised the orderliness of the demonstration, which he called "relatively well organized."

His sentiments were echoed by Joseph Yeldell, director of emergency preparedness at the Mayor's Command Center.

"It was a very good crowd, absolutely no problems," Yeldell said last night. "All of our efforts were dealing with moving people about rather than working about trouble. The mood was extremely good and supportive. I think it just turned out to be an excellent affair."

The 1963 march started off chaotically and Dr. Martin Luther King Jr. and other leaders ended up marching in the middle of the throng.

Organizers of yesterday's march, hoping to have more control over the flow, assigned colors to different contingents--purple for students, pink for senior citizens, brown for gay and lesbian groups--and floated balloons of those colors over areas where the groups were supposed to gather for the march.

However, the system functioned half-heartedly. There was much confusion over when the march actually began. Thousands of people deviated from the approved route, cutting across the Washington Monument grounds and proceeding down Constitution Avenue instead of along the Reflecting Pool.

U.S. Park Police said the crowd reached 250,000, while march sponsors estimated the crowd at 300,000.

Metro added 14 extra trains and 150 additional buses to handle the people, many of whom had been expected to arrive on 4,200 chartered buses and enter the Metro system at RFK Memorial Stadium and the Pentagon. But only about 1,800 of the expected buses actually arrived, easing the day's transportation problems.

Still, Metrorail trains were jammed through much of the day.

"If we had had to handle the people for 4,200 buses that they were talking about earlier, I think there would have been some real problems," Metro official Leroy Bailey said. Metro had to send some of its buses to RFK stadium to take pressure off the subway trains.

To avoid swamping the farecard system, Metro turned the machines off and locked open the fare gates. Patrons paid a flat 50 cents to ride the trains, dropping the money into metal barrels. At busy stations, officials controlled the number of people going down the escalators to avoid dangerous crowding on the platforms below.

"This is the best crowd I've ever seen," said Metro Police Chief Angus MacLean as he inspected the Smithsonian station, one of the busiest yesterday. "We haven't had one incident."

Metro officials reported the trains generally ran smoothly, though smoking brakes forced the evacuation of one train at the Smithsonian station. Metrobuses performed less well, with mechanical problems forcing the replacement of almost 100 vehicles during the day.

Most of the charter buses were to arrive on a staggered schedule by 9 a.m., allowing the march to begin at 10:30 a.m.

But by 10:10 only 314 of 3,000 expected at the stadium had come, and demonstration sponsors postponed the start of the march. Then, in the 70 minutes following 11:30 a.m., about 550 buses rolled into the stadium's parking lot, Bailey said..

However, in most cases detailed planning paid off.

Streets around the Lincoln Memorial and Mall demonstration site were closed to traffic and cleared of parked vehicles--24 vehicles had been towed away by 2 p.m. Early yesterday, city traffic lights were locked into morning rush-hour timings that favor inbound traffic to speed the buses' arrival.

To assist people on the Mall, 16 first-aid stations were set up around the demonstration site and fire department ambulances were on call. Officials estimated that more than 600 people altogether were treated for effects of the heat, and more than 100 people were treated for other minor injuries.

D.C. officials hooked up drinking water "bubblers" to fire hydrants at six sites. Vendors had trouble keeping up with the demand for cool drinks.

At the mayor's command post downtown, about 15 representatives from the police and fire departments, public health, the Civil Air Patrol and the Transportation Department monitored the profusion of radio channels in use and circulated news among themselves in an attempt to ensure coordination.

Though there were countless minor mix-ups during the day--people who got separated from parents or friends, National Guard water tanks that were sent to the Mall without water and buses that arrived at the District Building instead of the satellite parking sites--officials said the demonstration was one of the smoothest run in recent years.