Panamanian strongman Gen. Manuel Antonio Noriega, under increasing pressure from the United States to step down, flew to the Dominican Republic unannounced yesterday -- reportedly to attend a private party -- and word of his departure set off carnival-like celebrations in Panama that allegedly led to violent clashes with riot police last night.

Ricardo Arias Calerdon, president of Panama's opposition Christian Democratic Party, said truckloads of Panamanian Defense Forces troops broke up a demonstration after dark on a main thoroughfare of the capital. Troops fired tear gas and began beating and arresting demonstrators shortly after 7:30 p.m. after several hours of unhindered public celebrations of Noriega's departure, he said.

Arias said security forces fired their weapons during the operation, but the shots apparently were aimed over the heads of the demonstrators, and there were no reports of casualties. The crowds were dispersed by 9 p.m., he said. According to Arias, witnesses said dozens of demonstrators were arrested and that lawyers at night court confirmed the names of 15 detainees.

After denying during the day that Noriega had left the country, the Panamanian Defense Forces announced last night that he was on a private visit to the Dominican Republic and would "return to his official duties" on Monday. A television station controlled by the military repeatedly warned citizens to "remain calm" and ignore what it termed baseless rumors, a reference to beliefs that Noriega had given up power.

A clerk at Noriega's hotel in Santo Domingo confirmed last night that Noriega was in the Dominican Republic.

According to a senior U.S. official in Washington, the American Embassy in Panama has no evidence that Noriega fled the country or was giving up power, although the official said the embassy did not know with 100 percent certainty.

The embassy said Noriega had flown to Santo Domingo to attend a celebration in connection with his daughter's wedding, according to the official in Washington. His daughter was married in July to the son of a Dominican general. Sources in Washington who are part of the Panamanian opposition to Noriega said the general attended a reception last night given by his daughter's father-in-law.

Noriega's trip yesterday came after Reagan administration sources on Thursday revealed a secret mission to Panama the previous week by Assistant Secretary of Defense Richard L. Armitage to press Noriega to step down and allow free elections.

Figurehead President Eric Arturo Delvalle was also out of the country yesterday. The Panamanian Embassy in Washington said Delvalle has been undergoing medical tests in New York since Wednesday. Noriega is considered the power behind Delvalle's government.

The rumors in Panama that Noriega had stepped down originated with midday phone calls from opposition exiles in Washington saying Noriega had been spotted descending from a Panamanian Defense Forces plane in Santo Domingo, according to opposition sources in Panama City. From there, the opposition's highly active rumor mill churned into action.

Starting at about 4 p.m., thousands of Panamanians poured into the streets of Panama City in their cars and on foot to celebrate Noriega's departure. Panamanian sources in Washington said the crowds chanted, "Don't come back."

An hour later, a main boulevard that for months has been the major site of anti-Noriega demonstrations in the city reportedly was jammed for several miles with cars and with Panamanians honking car horns, banging pots and waving white handkerchiefs, the opposition's hallmark gestures.

Opposition leader Arias said similar celebrations took place in at least six other cities.

It was one of the largest demonstrations in Panama City since a political crisis erupted last June with charges by the Panamanian Defense Forces' former second in command, Col. Roberto Diaz Herrera, that Noriega had been involved in assassinations, election fraud and other crimes.

The clamor in the streets yesterday was so loud that it could be heard clearly over a long-distance telephone line. "The whole town is whipped up into a frenzy," a diplomat said.

"It's a huge carnival," said Ruben Carles, a publisher of the closed opposition newspaper La Prensa, in a telephone interview. "We don't know what the truth is about Noriega's trip, but the people are celebrating anyway."

"We hope he takes a permanent vacation," said Winston Robles, another La Prensa publisher.

Arias said many among the opposition hoped that the military would take a cue from the celebrations of Noriega's departure and prevent the general from returning to Panama. "Stopping him {Noriega} involves a military act," Arias said.

He said the opposition does not have plans for further demonstrations against Noriega, "just spontaneous combustion."

The demonstrations were particularly striking because yesterday was a national day of mourning in Panama, in memory of the deaths of 23 Panamanians during anti-American riots in 1964. The riots prompted Washington to negotiate with Panama over the Panama Canal. The negotiations led to the signing in 1978 of the treaty that gives control of the waterway to Panama in 2000.

U.S. concern about Noriega has increased with recent reports that he has made overtures to Libya and the Soviet Union in response to the freezing of U.S. aid and congressional moves to cut off economic and military assistance.

Correspondents Julia Preston in Managua, William Branigin in Mexico City and John Goshko in Washington contributed to this report.