MINSK, Belarus - The security services rounded up scores of opposition leaders and their supporters on Monday, a day after anti-government demonstrators attempted to storm the government headquarters here in a futile effort to block the re-election of Belarus's authoritarian president, Aleksandr G. Lukashenko.
Election officials said on Monday that Mr. Lukashenko received just under 80 percent of the vote as critics and outside observers called the election deeply flawed from the outset. At least six of the nine opposition candidates who ran against Mr. Lukashenko have been arrested, and police have been sweeping the city in search of their supporters.
"Arrests of the opposition are continuing," said Aleksandr Otroshchenko, an aid to one of the candidates, Andrei O. Sannikov, who was also arrested. Police had come to Mr. Otroshchenko's home Monday morning, he said, but he refused to allow them entry. "They will probably arrest me soon."
The independent rights organization Vyasna published an online list of 221 people it says have been arrested or prosecuted.
On Sunday, thousands of people converged on Independence Square here in the capital, heeding opposition leaders who called the day's election a farce and accused Mr. Lukashenko of keeping the post-Soviet country locked in dictatorship. The protesters chanted slogans like "Long Live Belarus" and disparaged Mr. Lukashenko, who in 16 years as president has muzzled the news media, eliminated political opponents and emboldened the secret police. At one point, protesters charged the entrance of the imposing government headquarters, breaking through glass doors and trying to push through barricades inside. But armored riot troops quickly overwhelmed the protesters, at times funneling them toward packs of plainclothes officers who beat them.
Earlier in the evening, one of the leading opposition candidates, Vladimir Neklyaev, appeared to have been beaten unconscious in a separate attack. He was taken to a hospital, but one of his aides said seven men wrapped him in a blanket and carried him away, as his wife screamed from a locked room, The Associated Press reported.
The United Stats Embassy in Minsk strongly condemned the violence. "We are especially concerned over excessive use of force by the authorities, including the beating and detention of several presidential candidates and violence against journalists and civil society activists," the embassy said in a statement on Monday.
The protests echoed similar demonstrations in 2006 after an earlier re-election of Mr. Lukashenko. Those protests were easily quashed.
By late Sunday evening, police officers wielding shields and clubs occupied large swaths of downtown Minsk. Earlier on Sunday, Mr. Lukashenko suggested that the authorities would take steps to ensure that the opposition would not be able to gather to protest the results. Speaking to reporters after casting his vote at a large athletic complex, he called members of the opposition "bandits and saboteurs."
"For half a year, people have been saying these elections will be unfair," he said. "Today my fate and theirs is being decided by the people of Belarus, and only the people of Belarus."
Yet even before the polls closed Sunday, opponents of Mr. Lukashenko began complaining of a police crackdown. Opposition activists said that several of their colleagues had been arrested by midafternoon, though it was unclear what the charges were.
Julia Rymashevsky, a spokeswoman for Mr. Neklyaev, said at least two aides had been arrested, including one who seemed to just disappear. "He called a taxi and left his apartment, but he never made it to the taxi," Ms. Rymashevsky said. Later, Mr. Neklyaev was headed to the central square to join the protest with about 100 or so followers when they were attacked without warning by men who wore black masks and clothing with no insignias. The men tossed stun grenades and began attacking people.
A reporter and a photographer for The New York Times were among those who were beaten, but they were not seriously injured.
The police slammed people to the ground and held them there for several minutes, pushing their heads into the snow, before suddenly leaving. Mr. Neklyaev appeared to have been knocked unconscious in the assault. After he was reportedly removed from the hospital, his whereabouts were unknown.
It did not appear that other opposition candidates were initially targets of the riot police on Sunday night. The large protest in central Minsk lasted for several hours before the police moved in.
"It was horrible," said Tatyana Molosh, a student. "I was barely able to get out. From one side the police were moving in, and then they came in from the other side. "But, you know what the most frightening thing is?" Ms. Molosh said. "I am 25 years old and I have to build a future and a family in this country and I see no future. This is the saddest thing."
Such complaints are nothing new in Belarus, particularly among the young, many of whom see little opportunity for success or growth in a country with an economy largely dependent on handouts from other countries.
The rising tensions on election night belied a concerted attempt by Mr. Lukashenko to make these elections appear more democratic in an effort to get money from the West as relations with his longtime patron, the Kremlin, have declined. Many Western countries, particularly in the European Union, have appeared heartened by these developments, and have sought to engage Mr. Lukashenko. Poland and Germany recently offered Belarus $3.5 billion in aid if this election was deemed free and fair.
In Minsk, however, few held out hope that it could be. "We have the ability to campaign and give people information," said Alesya Yakubouskaya, a student and opposition supporter. "But this is just so the government can show that it is liberal and that these elections will be fair."
"On the contrary," she said, "to think that the vote count will be fair is a little naive."