Like an unwelcome guest at a joyous family party, the Metro subway's balky Farecard system injected a jarring note of tension yesterday into an otherwise exhilarating day of inaugural celebrations.

The system was put to its greatest test by tens of thousands of riders heading for Ronald Reagan's swearing in, the inaugural parade and a fireworks display. Although the subway functioned smoothly at most times, it gave way at several key stations, leading to lines 25 deep, delays of an hour and more, and some potential danger as crowds overflowed platforms.

Passengers who had heeded official pleas to leave their cars at home poured into Metro stations only to encounter confusion and delay as throngs of out-of-towners and infrequent users grappled with the cumbersome Farecard process. In midmorning, Metro officials at several stations surrendered to the crush and threw open the gates, allowing tens of thousands of passengers to ride free. Similar crushes around 5 p.m. -- compounded by a 10-minute power outage on the Orange and Blue lines -- caused Metro to open the gates again for free passage.

"I've never seen anything like this before," said Master Robinson, the attendant at the Silver Spring station on the Red Line. "We've been letting them through by the thousands and thousands."

The problems that afflicted the subways constituted the only major blemish on an inaugural day that otherwise unfolded smoothly and according to plan. Aided by spring-like weather, a minimum of protest demonstrations and the fact that federal and city employes were granted a holiday, city officials were able to say that the day had passed without a hitch -- except for the subway.

Because of the holiday, Metro officials operated the subway and bus systems on a modified Saturday schedule, running 25 trains instead of the usual weekday 40 and 575 buses instead of 1,600. Crowds heading for inaugural events taxed the capacity of the buses on some major routes heading downtown, such as those on Connecticut Avenue NW, but service was reported generally adequate for the demand.

The subway, however, carried an estimated 160,000 passengers, about twice the normal Saturday total. The density of the crowds was compounded by the fact that most riders were traveling at the same time and using the same stations, and a high percentage were unfamiliar with the Farecard system.

On past occasions when large crowds were expected, such as the visit of Pope John Paul II in late 1979 and last Independence Day, Metro officials scrapped the Farecard system in favor of reliable buckets into which passengers tossed their fares. That move eliminated several time-consuming processes at once: the need to buy Farecards, line up at trunstiles, "go to Addfare" and insert the cards in turnstiles a second time to exit.

This time, according to Theodore G. Weigle Jr., Metro officials gave no consideration to adopting the bucket alternative because they believed the Farecard system would function adequately.

According to Weigle, it did. He said the movement of inaugural celebrators into the city was a "public transit success" that should not be judged by "minor glitches in the system." Under the circumstances, he said, "I would not have chosen to do things any other way."

The subway's major problems occurred at key stations in the hours between 9 and 11 a.m. as most riders headed for the inaugural ceremonies.

At the Pentagon station, passengers waited up to half an hour to buy a Farecard. When they reached the platform, they found it jamed from wall to platform edge, and some riders said they waited for as many as eight Blue Line trains to pass through before they found one with enough room to permit boarding. Some passengers rode the other way, to National Airport, where they were able to find seats for the return trip downtown.

Lines began forming at the Red Line terminal in Silver Spring before the trains started running at 6:30 a.m. By 10 a.m., with would-be Farecard buyers lined up 10 abreast for more than a block, station attendants threw the gates open to allow riders to board without cards, as they are authorized to do when they feel it necessary.

A similar scene unfolded at Dupont Circle, where lines 25 feet long formed at the machines.

At New Carrollton, the terminus of the Orange Line, the lines in front of the six Farecard vending machines flowed out of the station and back to the parking lot by 9 a.m. Station supervisor Joe Dempsey, discovering that three of the six machines were refusing to take dollar bills and the lines were getting longer, threw open the gates and allowed passengers to board free. "I don't think anyone envisioned this overwhelming crowd," he said. "Even if they [the vending machines] were all working 100 percent, we still couldn't handle this crowd."

Compounding the problems caused by the size of the crowds was the unfamiliarity of many passengers with the fare structure and the Farecard system. Not only out-of-town visitors had difficulties. Jody Scherner, who lives and works in Arlington but who seldom rides the subway, bought two 60-cent Farecards -- one for the trip downtown, the other for the trip back. She said she did not know that one card could be used for the combined fare.

At Capitol South, the nearest station to the site of the presidential swearing-in ceremony, the crowds overwhelmed the capacity of the escalators and exit turnstiles to let them out. At about 9:30 a.m., with trains of the Blue and Orange lines pouring in to discharge passengers, attendants turned off the escalators because, they said, more passengers can be moved with the escalators halted. Still the crowds below grew, until the entire 550-foot platform was filled track-to-track with passengers trying to get out. Arriving trains began waiting with their doors closed until space was cleared on the platform for their riders to disembark. Meanwhile, a few passengers trying to board at the station struggled to get down the halted escalators to the platform.

Above ground, surface traffic generally moved smoothly except in the area immediately around the parade route, where chartered buses, limousines and police vehicles contributed to traffic snarls. City officials reported that 75 to 100 illegally parked cars were towed away -- including two that belonged to the Secret Service.