A good-natured crowd of about 300,000 persons, many arriving by bus and subway, gathered in cool, windy weather on and around the Mall last night to gape and gasp at the city's rain-delayed Independence Day fireworks display.

Within a few minutes of the end of the show at 9:45 p.m., subway stations in the Mall area were jammed with a hugh crush of homebound passengers taking advantage of free service.

In little more than an hour, the platforms were empty - to the great delight of Metro officials. "It was fantastic," said Metro assistant general manager Nicholas J. Roll. "I've never seen anything move so smoothly."

The surprising number of subway riders who continued to arrive at Mall stations while fireworks were already bursting overhead and even after the last burst faded, contributed to an incident at the Smithsonian station that was the evening's major snarl.

Just as the show ended at 9:45 p.m., a swelling stream of persons attempting to leave the Mall found themselves suddenly bucking a strong tide of persons still climbing out of the station's Mall entrance. The turbulent and unexpected confrontation produced shoves and shouts and screams, but it ended without report of injury.

For the most part, the $20,000 fireworks spectacular, postponed from Wednesday because of rain, provided the excuse and occasion for an evening of peace, calm, order and cheery good spirits.

In traditional summer shirtsleeves, or in sweaters and jackets to ward off the unseasonably cool 70-degree temperature and 14-mile-an hour winds, they picnicked, flung Frisbees and set off portable pyrotechnic devices of their own while waiting for the show to start.

Sedate suburban couples with small children, T-shirted teen-aged couples holding hands, they sat upon the still damp greensward, passing the time in often-topical chatter.

"Wouldn't it be funny," suggested one grisly minded child, "if Skylab came crashing down on the Monument grounds."

Wryly, surveying the happy sea of humanity that swelled around him, one Washington resident was reminded of another current American concern.

"I haven't seen a crowd this big since I tried to get gas on Tuesday," observed Mike Troy.

On bicycles, carrying blankets, toting coolers of beer, the crowd continued to swell and spread as the strains of blueglass music came over loudspeakers and the sun sank in a brilliant burst of color behind the dark ridge of Arlington in the west.

The pastel orange glow from the sunset was still tinging the high thin clouds overhead when the fireworks display began.

With its hundreds of brilliantly colored aerial bursts, the slow, an Independence Day tradition in Washington, evoked the traditional sighs and shrieks of admiration and excitement from the crowd, which was spread over hundreds of acres of parkland around the Washington Monument.

Part of the show also gave spectators a few moments of apprehension. Near the show's close, two rockets appeared to detonate prematurely, and debris sifted down towards the watching crowd. No injuries were reported.

Fireworks impresario Felix Grucci Jr, who masterminded the show, said the rockets' lift charges had been dampened by Wednesday's heavy rain.

Taking into consideration an estimated 40,000 persons who watched from vantage points along the George Washington Memorial Parkway on the west bank of the Potomac River, Park Police put the total crowd at 310,000.

One of the largest gatherings - 60,000 persons - was on the Capitol grounds where the National Symphony Orchestra gave an 8 p.m. concert of patriotic and martial music. Like the fireworks, the concert had also been postponed from the Fourth.

Also heavily populated were the immediate environs of the Smithsonian Metro Subway station, the only terminal on the Mall.

After emerging from the station, many persons showed little desire to go farther, apparently intending to be in the front rank of the great homeward exodus to come.

Metro officials appeared to regard their performance during and before The Great Exodus as one of their finest hours.

In the Silver Spring station 4,000 persons were lined up at farecard machines at 7:45 p.m., according to Metro's general manager, Richard S. Page. Similar figures were reported at other outlying stations. "It was like morning rushhour," another Metro official said.

Officials had planned to begin offering free subway and bus service at 9 p.m. The long lines prompted them to throw open the subway faregates an hour earlier.

It "was like breaking the fever," Page said.

A record number of trains was thrown into service. Metro spokesmen said 35 trains were in simultaneous operation on the Red and Blue lines, four more than ever before.

Buses, which began offering free service at 9 p.m. as scheduled, also carried heavy loads and ran smoothly, according to Metro spokesman Cody Pfanstiehl. He said full ridership figures were not immediately available.

The National Park service also seemed satisfied with the evening's events. Crime was "real low" in the fireworks watching area, said Park Service spokesman George Berklacy.

He said two robberies were reported on the Mall and one on the Monument grounds. No serious incidents of assault were reported, he said.

In addition, he said, there was relatively little debris. "Usually we can tell by the grim face of the chief maintenance officer," Berklacy said. "He's smiling." CAPTION: Picture 1, After last night's Independence Day fireworks show, persons leaving the Mall found themselves jammed into a crowd at Smithsonian subway stop. By Lucian Perkins - The Washington Post; Picture 2, Last night's fireworks, as seen from the top of the Lincoln Memorial. Part of the crowd gathered around Reflecting Pool at Washington Monument. By John McDonnell - The Washington Post