Belarus has blocked access to social networking sites in an attempt to prevent protests being held on a national holiday, rights activists say.
The opposition also says dozens of activists have been detained.
Speaking in Minsk, President Alexander Lukashenko denounced what he said were efforts to overthrow his government.
Activists had been using social media sites to organise protests for Independence Day, the anniversary of the end of Nazi occupation in 1944.
Protests are rare in Belarus, where the authoritarian rule of Mr Lukashenko has led to sanctions by Western governments.
Demonstrators had been urged to attend the celebrations and start clapping as soon as Mr Lukashenko began his Independence Day speech.
'Titanic effort'The opposition have held a series of internet-organised marches known as "Revolution by Social Networks", in which protesters do not carry signs but instead walk through the streets clapping in unison.
The group's main page on a Russian social media site was blocked, with activists saying it was a deliberate attempt to stifle their efforts.
"The authorities are making a titanic effort to break the wave of civil protests," wrote opposition website Charter 97.
"Start Quote
End Quote Stanislav Shushkevich Opposition memberThese are clearly illegal actions undertaken to prevent the protest actions called for on the internet"
Speaking ahead of a military parade, President Lukashenko - dressed in military uniform - said rallies co-ordinated on the internet were part of a plan to overthrow his government.
"(Somebody) is trying to copy a 'coloured revolution' scenario here," he said, referring to protest movements in ex-Soviet republics such as Georgia and Ukraine in 2003-2004.
"They want to put us on our knees. This is not going to happen."
At least one person who began clapping was quickly led away by plain-clothes policemen, according to Reuters news agency.
Valentin Stefanovich from rights group Vesna said the government had detained dozens of activists, while others had been called in by the KGB and warned not to protest.
Those arrested included Stanislav Shushkevich, the first post-Soviet leader of Belarus and a strong opposition supporter, but he was later released.
"These are clearly illegal actions undertaken to prevent the protest actions called for on the internet," Mr Shushkevich told AFP news agency.
Last month, about 1,000 people gathered in Minsk to take part in a "silent protest" over the economic crisis organised via social networking sites.
In December 2010, authorities cracked down on demonstrations held after general elections to protest against alleged vote-rigging.
International monitors said the contest, in which Mr Lukashenko officially won 80% of the vote, was deeply flawed.
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