Alexander Milinkevich was sentenced on Thursday along with three other opposition politicians after being found guilty of leading a rally that was deemed illegal by the government.
Milinkevich was arrested the day after a demonstration that attracted about 7,000 protestors and coincided with the 20th anniversary of the Chernobyl nuclear accident.
Looking calm as the judge read out the sentence, Milinkevich denied he was guilty of any crime.
"This is a political action, a political sentence," he told the court. "Leaders of leading political parties are behind bars."
The other three people sentenced after the rally were Vintsuk Vyachorka, leader of the Belarusian People's Front organisation and a former official in Milinkevich's unsuccessful presidential campaign; Alexander Bukhvostov, leader of the opposition Trud party; and Sergei Kalyakin, Milinkevich's campaign manager.
Lukashenko has been accused by
the West of crushing dissent
Milinkevich has become a focus for opposition to Alexander Lukashenko, the president, accused in the West of crushing dissent.
The EU said Lukashenko's landslide re-election last month ahead of Milinkevich was rigged. He has already imposed a travel ban on the Russian-backed Lukashenko and 30 key ministers.
It demanded the immediate release of the opposition leader.
Emma Udwin, a European Commission spokeswoman, said: "We are following this extremely closely. We condemn any detentions that are for reason of having taken part in a demonstration or any other political activity."
Condoleezza Rice, the US secretary of state, who was in the Bulgarian capital, Sofia, for a Nato meeting, said the US "roundly condemns this act and sincerely hopes that the Belarussian government will accept the will of the international community".
She called on the Minsk authorities to "act within accepted international principles when it comes to the treatment of political opposition".
The Nato chief, Jaap de Hoop Scheffer, who was also in Sofia, condemned the decision.
"This is of course unacceptable," he said. "I call on the regime in Minsk to release him immediately... The Euro-Atlantic community cannot accept this."
The government deems many
rallies as illegal
More than 1,000 opposition activists in Belarus have been arrested since the presidential election last month, most of them given sentences of one to two weeks for attending unsanctioned rallies.
According to Milinkevich, Wednesday's protest was not illegal because the crowd of about 6,000 gathered at a place allocated to them by the city authorities.
At the demonstration, Milinkevich urged supporters to work for the constitutional overthrow of Lukashenko.