MINSK (AFP) -- An opposition leader in Belarus was detained for several hours on Thursday after he attempted to confront the nation's hardline President Alexander Lukashenko at a political convention in Minsk, police said.

Alexander Kozulin, leader of the Social Democratic party and one of four candidates in a presidential election slated for March 19, told AFP he was held in a police station and released later Thursday.

Witnesses said Kozuli,n a former dean of Minsk university, was beaten up after organizers refused to register him when he arrived at the Congress of People's Deputies demanding to be let in.

Prosecutors said Kozulin was under investigation for "hooliganism," which is punishable by up to 15 days in prison, and that he could incur an additional penalty for smashing a portrait of Lukashenko.

The detention set off alarm bells in Western capitals ahead of the forthcoming presidential election.

The U.S. administration urged the country's hardline regime to allow free elections or face consequences. "We have indicated to them that should there be incidents like there were today, there will be consequences," David Kramer, deputy assistant secretary of state for European and Eurasian affairs, told reporters.

"We are paying very close attention to those who are involved in activities that promote either fraudulent elections or promote violence," added Kramer, who was in Minsk last week to meet with government officials as well as members of the opposition ahead of the elections.

In Vienna the Organization for Security and Cooperation in Europe (OSCE), which plans to monitor the presidential election, also denounced the detention and demanded an explanation from the Minsk authorities.

"This marks a serious deterioration in the campaign atmosphere," it said in a statement.

In Brussels the European Union described the detention as a "serious development".

Kozulin has strongly denounced Lukashenko as the heated election campaign gets under way in the former Soviet republic, where almost all opposition media have been shut down.

During a television broadcast, he ripped up an issue of the pro-government Sovetskaya Belarus newspaper devoted entirely to Lukashenko.

At the convention centre, Lukashenko -- whose regime has been described by U.S. Secretary of State Condoleezza Rice as "the last dictatorship in the center of Europe" -- railed against what he called outside interference.

"If we give up our country without fighting, our descendants will never forgive us," said Lukashenko, who is widely favored to win the election.

He vowed to put down any attempts at an "orange revolution" similar to the popular street demonstrations that brought pro-Western President Viktor Yushchenko to power in neighboring Ukraine.

In front of 2,000 people, including many government officials and military personnel, Lukashenko accused the United States and the European Union of financing the opposition.

"They don't have any lessons to teach us in terms of human rights. They've covered the Middle East, Afghanistan, Iraq, Yugoslavia in blood.

"They say: We're going to democratize Belarus. Democratize yourselves first!" said Lukashenko, who organized a controversial referendum in 2004 to enable him to be re-elected in perpetuity.