Alexander Kozulin (left) wanted to speak out against the president

The man, Social Democratic Party leader Alexander Kozulin, was also beat up by police in Minsk, witnesses said.

He was detained after trying to gain entry to a conference being addressed by President Alexander Lukashenko.

Thousands of supporters of another opposition candidate, Alexander Milinkevich, held a rally in Minsk.

There was a stand-off as the streets were filled with riot police. The authorities had said the rally was illegal.

The crowd later dispersed peacefully.

Police earlier prevented Mr Kozulin, 50, attending a Soviet-style All-Belarussian People's Assembly staged by President Lukashenko.

"I wanted to tell the truth about the dictatorship we live in," Mr Kozulin was quoted as saying.

International election monitors say they heard gunshots outside the police station where he and several supporters were detained.

Soviet icons

The Belarussian news agency Belapan reported that more than 20 Kozulin supporters were detained by police later.

Congress delegates are likely to praise Mr Lukashenko's leadership

The BBC's Steve Rosenberg, in Minsk, says the two-day congress is President Lukashenko's version of Communist congresses that used to be held in the USSR.

At the congress President Lukashenko accused the West of trying to interfere in the Belarussian election to provoke a transfer of power like neighbouring Ukraine's Orange Revolution.

"The strategy of the West and the domestic opposition is to drag Belarus backwards," he told the delegates.

The assembly is just one of many icons of Soviet life revived by President Lukashenko in the 12 years he has been in power, our correspondent says. Among the others are the old Soviet-era Belarussian flag and anthem, the command economy and the police state.

He rules Belarus with an iron hand - Washington has condemned him as "Europe's last dictator".

Mr Lukashenko, who is bidding for a third term in office in the elections, has said he will not be doing any traditional campaigning.