International observers and Western governments accused Belarus’ strongman leader of using fraud and violence to remain in power after more than 16 years of repressive rule, saying on Monday that President Alexander Lukashenko’s re-election had been seriously flawed.

The Organization for Security and Cooperation in Europe said the count in Sunday’s vote was “bad or very bad” in half the country’s precincts. It also strongly criticized the violent dispersal by riot police of a post-election protest rally. U.S. and European leaders criticized Mr. Lukashenko for a wave of violence directed at rival presidential candidates and their supporters in the hours after the election.

Tens of thousands of protesters gathered late Sunday to denounce alleged fraud in the vote. Police beat and detained hundreds and arrested seven of the nine candidates opposing Mr. Lukashenko. One of the top opposition candidates, Vladimir Neklyayev, was beaten in a clash with government forces as he tried to lead a column of supporters to the protest. He was taken to a hospital, but his aide said seven men in civilian clothing later wrapped Mr. Neklyayev in a blanket on his hospital bed and carried him away as his wife screamed.

The country’s election commission declared on Monday that Mr. Lukashenko got almost 80 percent of the vote in a preliminary count, handing a fourth term in office to the man who was run the former Soviet state with an iron hand since 1994.

Mr. Lukashenko has allowed no independent broadcast media, kept 80 percent of industry under Soviet-style state control and suppressed opposition with police raids and pressure.

The election had raised a glimmer of hope that Mr. Lukashenko was relaxing his grip. The number of candidates was unprecedented, they were allowed comparative freedom to campaign and were even allotted time for debates on state media. Belarus had also passed some reforms in its election code.

But “this election failed to give Belarus the new start it needed. The counting process lacked transparency. The people of Belarus deserved better. And, in particular, I now expect the government to account for the arrests of presidential candidates, journalists and human rights activists,” said Tony Lloyd, one of the mission leaders.

The U.S. Embassy said that Washington “strongly condemns all election day violence in Belarus.” German Foreign Minister Guido Westerwelle said that “it’s not acceptable to harass, beat or arrest opposition candidates and their supporters who want to exert their right to freedom of expression.”

The strong critique is likely to renew tensions between Belarus and the West, which had ebbed over the past year as Mr. Lukashenko appeared to be incrementally reforming. He had released some political prisoners and had made Belarus part of the European Union’s Eastern Partnership initiative.

It could also bolster the morale of opposition backers, dismayed by the arrest of most of their leaders and searching for a strategy to keep their protests in the spotlight.

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