Amid falling snow and massive traffic jams, America's 50th presidential inaugural festivities opened with a bang of fireworks on the Mall last night, as thousands of Republican faithful streamed into the capital to celebrate the start of President Reagan's second term.

As the four-day pageantry began, the demand for downtown hotel rooms, limousines and tuxedos was exceeded only by the enthusiasm of the celebrants, the frustration of motorists maneuvering around sealed-off streets, and the zeal of the thousands of law enforcement authorities assigned to guard the participants.

At the opening patriotic pageant, D.C. police and the Secret Service -- aided by metal detectors erected near the Ellipse -- stood watch over the crowd of thousands who stood in subfreezing temperatures to hear the largest assemblage of fife and drum corps and buglers this side of July 4th.

The combination of the crowds streaming to the Ellipse, the blocked off streets and the evening rush hour created what harried police officers called one of the worst traffic jams in memory.

The festival of Americana, performed on a stage decorated with an eagle with a 40-foot wingspan, was witnessed by Reagan and Vice President Bush from a heated, bulletproof enclosure. There were Marching Blue Devils from Brunswick, Ohio, performers and appreciative listeners from across the nation, secretaries from the Treasury Department, a senator's daughter swathed in mink, and parents clutching babies in one arm and blankets in the other.

Spirits were high before the start of the 22 official inaugural events and hundreds of semiofficial and unofficial gatherings. "I am thrilled. I was very happy the last time he was elected, but this is even a happier time," said Blanka Krizek, wrapped in a fur coat and hat, who was waiting to buy $200 tickets for herself and her husband George, a Washington psychiatrist. Krizek, a Czechoslovak immigrant who fled her homeland in 1968, said, "We have lived through the horrors of communism, so at a time like this, you appreciate . . . America."

For others, though, the second Reagan inauguration was basically a winter rerun, lacking the drama of 1981, when the triumphant GOP ousted the Democrats from the White House while America held its breath awaiting the word on freedom for 52 hostages in Iran. That word came minutes after Reagan concluded his first inaugural address.

"This time is like getting married a second time. . . . There's kind of a deflation," said Robert Ross, 63, a retired New Jersey restaurateur attending his third GOP inauguration with his wife Claudia, a GOP voter registrar in Bergen County. The Rosses, who wore matching black fur coats, are spending $1,200 on tickets and $800 on their hotel, and they expect their weekend tab to exceed $3,500 -- including their $1,000 membership in the Republican Senatorial "inner circle," which earned them the right to buy $200 tickets to the presidential gala.

The Presidential Inaugural Committee has billed the 1985 festivities an inaugural for ordinary people, partly in response to criticisms that the 1981 Reagan event was an elitist affair geared to the very rich, who forked out as much as $1,500 a ticket. The 1981 inauguration -- produced by Reagan's Hollywood friend, Charles Z. Wick, director of the U.S. Information Agency -- ran a record-high price tag of more than $16 million. This year's planners limited ticket prices to $200 and the budget to about $12 million.

The scaled-down prices put the affair within the budget of people such as Richard and Dawn Smith of Cherokee County, Okla. ("near Muskogee"), who drove 1,200 miles with her parents and sat in their dungarees at the D.C. Convention Center waiting for Dawn's mother to work out problems with tickets.

"We are not rich people," said Dawn Smith. "It seems like everyone around here wears a fur coat, even the men."

Smith's mother, Joann Bradley, chairman of the Republican Women of Cherokee County, had charged eight tickets to her American Express card, she said, but found only five waiting when they got here. Their problem, like that of many others, was remedied following some anxious waiting.

But such glitches in the computerized ticketing operation had no respect for position. When Kit Mehrtens, the newly elected secretary of the Republican National Committee, ordered her tickets Dec. 19, she expected to get them before leaving her Arizona home. When they did not come and she left for .ashington, she figured that her $550 worth of tickets would be waiting. Wrong again.

"I still don't have them," she said yesterday while waiting for Bush to arrive at an RNC luncheon. "But I'm going to keep the faith."

In the luxury of the J.W. Marriott Hotel's ballroom, the Republican hierarchy gathered for a luncheon where the U.S. Senate chaplain, the Rev. Richard C. Halverson, thanked the Lord "for the overwhelming victory in November."

Everybody, it seems, was throwing an invitations-only party for the Grand Old Party: from the Guam Society of America, to Athletes for Reagan-Bush, to the International Brotherhood of Teamsters, to much of the Fortune 500. And anybody who wanted to look like somebody was renting a tuxedo for the occasion, and a limo to get there