KWANGJU, SOUTH KOREA, DEC. 19 -- More than 5,000 angry students demonstrated today near a park in the center of this opposition stronghold, chanting slogans against president-elect Roh Tae Woo and against the United States government.

The rally appeared to be the largest and best organized protest since Wednesday's election.

The demonstration was mostly peaceful, and hundreds of riot police appeared to keep their distance during the rally, which began in the late afternoon.

By nightfall, most of the protesters had dispersed, but the few hundred who remained hurled fire bombs and chunks of concrete at police, who responded by firing dozens of volleys of tear gas.

The protesters used bricks and metal pipe to blockade several city blocks near the entrance to Kwangju Park, where they lit bonfires, ripped up the concrete pavement, unfurled yellow and red flags with antigovernment slogans and chanted the name of this city's local hero, defeated opposition candidate Kim Dae Jung.

The protest, organized by students of Chonnam National University, also carried a decidedly anti-American tone.

The protest leaders distributed fliers that criticized the State Department and Rep. Stephen J. Solarz (D-N.Y.) for calling the election fair, and warned the United States and Japan "not to continue supporting the military regime."

{In his Saturday radio address, President Reagan urged South Koreans to accept the election result. "The most important victory is for democracy," Reagan said.

{"As Americans know, and as Koreans are finding out, elections have losers as well as winners," he added. He urged the opposition "perhaps to try again at the next election."}

The antigovernment rally was clearly the largest here in Kwangju and perhaps in the country since ruling party candidate Roh, a former general and the handpicked successor of President Chun Doo Hwan, was elected to a five-year term with 36.6 percent of the vote.

Opposition supporters claimed the government rigged the election for Roh, although independent observers said that incidents of voting irregularity did not appear widespread.

In Seoul, more than 1,000 protesters staged a rally at Myongdong Cathedral, saying the election should be voided because of widespread government cheating.

Roh, who met with U.S. Ambassador James Lilley to receive a congratulatory message from President Reagan, said he hopes to hold National Assembly elections about Feb. 10, before his Feb. 25 inauguration.

It remained unclear whether the opposition parties, which have rejected Roh's election and refused to concede, will participate in negotiations to revise the election law needed before such a vote can take place.

The Kwangju protesters had hoped to emulate popular protests that occurred in the Philippines in February 1986, when then-president Ferdinand E. Marcos was declared the winner of an election marked by widespread evidence of fraud and vote manipulation.

Marcos was overthrown three weeks later when a breakaway faction of the military joined forces with the "people power" protesters in the streets.

The election here, however, has won international praise as having been generally fair, and the protests have failed to attract significantly broad-based support.

Today's rally appeared confined to university students and young people -- many of them teen-agers -- and seemed to attract little, if any, support from other Kwangju citizens.

The protest was much smaller than violent demonstrations in June that led Chun to accept a package of democratic reforms, including direct election of the president.

Some analysts here say the opposition must attract support from the mainstream, particulary the middle class, if their demonstrations are to escalate into a significant challenge for Roh's incoming government.

Protest organizers said, however, that they first need a clear signal from Kim Dae Jung before they can convince masses of citizens to join them in the streets.

Even if Kim were to call for mass action, it was unclear whether a majority of his supporters would follow, even here in his home province.

"Supporting Kim is different than following what he says," said a reporter for the local afternoon newspaper, The Kwangju Daily, who asked not to be quoted by name.

"People are angry with the election result, but I don't think the demonstrations will continue for a long time. I think citizens can accept the election result."

"The students are free to protest," said another resident and Kim supporter, taxi driver Yang Sun Ho. "Ordinary citizens have other things to think about -- like their families." He watched the rally from the roof of his house on a hill overlooking the park.

As the drive to mount a successful, nationwide protest movement appears to fizzle, anti-American sentiment appears to be growing, particularly after the strong words of support for Roh from the White House, the State Department and Solarz.

American media representatives have been singled out as targets for attack, especially since a government-owned television station began broadcasting names of American and other foreign newspapers that it said have endorsed the election results.

Sporadic attacks also continued today on foreign journalists. A Japanese television crew left the rally scene after being pelted with chunks of concrete, and a cameraman said he was hit on the back of the head and shoulder.

"The students think the United States was too deeply involved in the Korean election," said Yang, the taxi driver. "They are very upset about the Americans' behavior."