SEOUL, DEC. 19 (SATURDAY) -- More than 4,000 riot police stormed a municipal office yesterday to wrest four disputed ballot boxes from more than 1,000 students and opposition supporters.

The assault led to a two-hour clash in which 64 people were injured, 1,005 protesters were detained and the five-story office building was nearly gutted. The operation also removed the one piece of physical evidence that the opposition had said could prove its contention that the government committed widespread fraud in Wednesday's presidential election.

A handful of clashes took place elsewhere in Seoul and around the country as students protested Wednesday's victory by ruling party candidate Roh Tae Woo. But they mostly involved small numbers of protesters who were quickly dispersed.

According to witnesses, students began occupying Myongdong Cathedral in downtown Seoul last night, and by early this morning about 600 protesters were inside the church, the site of key protests in June that pressured President Chun Doo Hwan to allow the elections.

One of the largest protests yesterday took place in Kwangju, stronghold of defeated opposition candidate Kim Dae Jung, where police lobbed tear gas grenades to scatter about 1,000 students and other protesters after a two-hour confrontation. Opposition leaders there expressed frustration at their inability to stage a larger demonstration that had been planned to protest what they called a fraudulent election.

In Kwangju, anti-American sentiment and resentment at the foreign press increased among a small number of students as they learned that U.S. officials and newspapers had urged acceptance of Roh's election. An NBC television crew was roughed up, and a car carrying three U.S. reporters was attacked, but there were no serious injuries.

Two international observer missions, one of which included several congressional aides, said they had witnessed abuses favoring the ruling party during Wednesday's voting. Both groups said they could not conclude that the election was systematically unfair, but both said the abuses were disturbing and should be investigated.

Roh won the election with a 36 percent plurality when two opposition leaders, Kim Dae Jung and Kim Young Sam, split the antigovernment vote. The two Kims then said the government cheated so massively before and during the election that the result should be discarded.

The government officially declared Roh the winner yesterday and provided him with security guards and a bulletproof Cadillac. President Chun, who came to power with Roh in a 1979 coup and who has promised to step down next February, said yesterday that the two Kims "should heartily congratulate the winner."

According to the final count, Roh won 8,282,738 votes, Kim Young Sam 6,337,581, Kim Dae Jung 6,113,375 and Kim Jong Pil 1,823,067.

Chun also warned that the government will now crack down hard on protests. He said the regime had been lax during the campaign "to ensure a free atmosphere."

"I realize that because of this caution, many citizens had to suffer anxiety and inconvenience," Chun said. "Accordingly, the administration will now sternly deal with any and all illegal and disorderly acts."

The ruling party continued to express confidence that most Koreans will not support the demonstrations. Storekeepers and pedestrians watching the sporadic rallies yesterday more often expressed disgust or impatience than support.

But emotions continued to run high among some student groups, dissidents and opposition workers, reflecting the country's sharp divisions exacerbated by the election.

Many dissidents view Roh as a "murderer" for his part in the coup that brought Chun to power -- in reaction to which hundreds of protesters in Kwangju were killed -- and they say they can never accept him. A coalition of dissident groups agreed yesterday to form the "Pan-National Conference for the Invalidation of the Fraudulent Election."

Wild rumors circulating among the dissidents yesterday here and in Kwangju fueled those emotions -- that paratroopers had landed in Kwangju, for example, or that dozens had died in the raid on the municipal office here. Distrustful of the progovernment media, many protesters seemed inclined to believe almost anything they heard.

Several suicides and suicide attempts further heightened tension among the opposition.

In one case, Ho Ki Soo, 38, the government's block captain in a southeastern Seoul neighborhood, set himself on fire on a rooftop after yelling that he opposed unfair elections and that he had received $30 before the election, presumably in exchange for his vote or his neighbors'. Onlookers extinguished the flames and took him to the hospital, where he remained in critical condition, police said.

In another instance, an opposition party local chapter head in North Chungchong province, Chung Jae Sok, committed suicide by inhaling carbon monoxide, police said. Chung's suicide note urged the two Kims to accept the election result and engage in "self-reflection."

The massive police assault on the municipal office in the Kuro district of Seoul also heightened tensions among dissidents. The incident there began on Election Day, when a Kim Dae Jung supporter apparently found a green strongbox, used for transporting ballots, hidden under some bread and other food on a truck in front of the building.

A number of Kim Dae Jung supporters seized the box, believing that it contained false absentee ballots. Supporters eventually occupied the five-story building, seized three more boxes and held a number of officials hostage, according to police. By this morning, between 1,000 and 2,000 protesters had barricaded themselves in the building.

The opposition, which has claimed widespread fraud in Wednesday's vote but has produced little hard evidence, believed that the boxes would help prove its case.

Government officials said there was nothing suspicious about the boxes. They said officials had been taking them to the counting station, along with snack food, and that poll-watchers did not have to accompany the box, because the absentee ballots inside were double-sealed.

Because police removed the boxes during the melee yesterday, the dispute likely will never be resolved. The black-helmeted riot police attacked at about 6 a.m., first spreading 100 mattresses on the ground around the building and then firing tear gas inside.

At least one protester, Yang Won Tae, 23, a Seoul student, fell or jumped from the fifth floor. Police said Yang's legs were broken. His brother-in-law said doctors told him that Yang's spine was broken in a way that might leave him paralyzed. Other, unconfirmed accounts had others also falling from the roof.

During the fight, protesters set fire to office furniture and threw bricks and molotov cocktails, while police wielded clubs and tear gas. More than 100 people took to the roof, hurling tiles at police who approached by fire-truck ladder.

Election officials said they took the boxes to headquarters but, because accompanying documentation had been burned, would not count the votes.

The reports from the two international observer groups raised questions about Wednesday's election but did not attempt to gauge whether fraud could have accounted for Roh's winning margin of 2 million votes.

The International Human Rights Law Group, in fact, was fairly upbeat about the election process.

"What we witnessed was a relatively well-organized and efficient ballot," its report said. "Even if the possibility of dubious and tainted votes remains open, we did not witness specific cases of fraud or serious electoral mismanagement."

However, the group faulted the police presence outside some polling stations because it said it created "an atmosphere of intimidation." It criticized the lack of control over absentee ballots. Its report also said the ruling party enjoyed an "unfair advantage" during the campaign, especially in media coverage.

A separate delegation of Korean experts and U.S. congressional aides was less upbeat. That group, led by Jan Kalicki, a former aide to Sen. Edward M. Kennedy (D-Mass.), and Andrew Semmel, an aide to Sen. Richard G. Lugar (R-Ind.), said it had witnessed numerous violations but was "unable to say how widespread these practices have been."

The violations, the delegation said, included counterfeit ballots, multiple voting and the beating of Korean election observers. Washington Post correspondent Keith B. Richburg contributed to this article from Kwangju.