The main opposition leader in Belarus, Alexander Milinkevich, has been jailed for 15 days after being convicted of attending an unsanctioned rally.
Alexander Milinkevich had called for a peaceful protest
He had been charged with breaking the law by staging a rally on Wednesday in the centre of the capital, Minsk, instead of on the outskirts.
Mr Milinkevich said the move to jail him for the Chernobyl 20th anniversary march was a "political sentence".
US Secretary of State Condoleezza Rice said the decision was "reprehensible".
She said she hoped the Belarussian government would "act within accepted international principles when it comes to the treatment of political opposition", the AFP news agency reports.
Three other anti-presidential activists were also jailed following the march.
Earlier, Mr Milinkevich said he believed he had taken part in an authorised event.
Protesters had asked for permission to demonstrate in two places, he told AFP news agency.
"We were authorised to demonstrate in one of them and that is where we went," he said.
The Chernobyl anniversary is a traditional forum for the opposition
Mr Milinkevich's spokesman said the opposition was already preparing an appeal.
The leader himself told reporters through the barred window of the courtroom he did not feel intimidated and was not going to give up his political struggle.
Activists said veteran opposition leader Vintsuk Vyachorka was also sentenced to 15 days in jail for public order offences.
Senior opposition figures Alexander Bukhvostov and Sergei Kalyakin were jailed for 15 and 14 days respectively.
Earlier on Thursday another opposition activist, Anatoly Lebedko, described how he had been beaten up, handcuffed and blindfolded by men he described as security officers.
He said he had then spent five hours in an interrogation chamber run by the Belarussian secret police.
At least 6,000 protesters rallied in Minsk for the march to mark the Chernobyl nuclear disaster in Ukraine. The anniversary march is a traditional forum for Belarussian opposition.
During the event a few hundred of Mr Milinkevich's supporters defied police orders and carried their banners through the city centre.
Thousands of riot police accompanied them, and there were official warnings about severe penalties for any unsanctioned demonstrations.
Mr Milinkevich was briefly detained before the rally and told to stay away from October Square, Reuters news agency said.
Demonstrators had initially tried to gather at the square, but later moved to another site which had been authorised by the authorities, the agency added.
The square was the scene of demonstrations following President Alexander Lukashenko's landslide victory in last month's elections.
The polls brought Mr Lukashenko a third term and widespread condemnation from the international community. Observers said the polls were neither free nor fair.
Belarussian courts jailed more than 150 opposition protesters following the resulting demonstrations.
Ahead of Wednesday's Chernobyl march, Mr Milinkevich had repeatedly asked his supporters to avoid any confrontation with the security services.
He told the BBC the opposition wanted the march to be peaceful.
At the rally, he told protesters the opposition planned to oust Mr Lukashenko within two years through "peaceful action".
He called for another rally to take place on 1 May.
The European Union said it was following the arrests in Belarus closely.
"We condemn any detentions that are for reason of having taken part in a demonstration or any other political activity," said European Commission spokeswoman Emma Udwin.