Belarus has jailed several protesters who were among hundreds arrested in a nationwide crackdown during protests on Sunday against President Alexander Lukashenko's government.
By late Monday, 20 people had been handed jail sentences of up to 15 days in prison while dozens of others awaited their turn in packed courtrooms. Several hearings were put off to Tuesday.
"The authorities must immediately free all those detained and ensure freedom of assembly for citizens and stop the shameful practice of beating and detaining the participants in rallies," Vyasna, a Belarusian human rights centre, said.
The rights group said 210 people were arrested in the capital Minsk where hundreds took part in the protests, while another 180 were seized in other parts of the country, including the regional centres of Grodno, Gomel and Mogilev.
Responding to calls for a rally by an Internet-based group "Revolution through the Social Network", the protesters defied warnings and state jamming of websites to turn out in reasonable numbers.
They sought to show their dissatisfaction with Lukashenko by clapping their hands. But police arrested anyone who joined in the applause, beating activists and firing tear gas, an AFP correspondent in Minsk said.
Most of the arrests were carried out by police in plain clothes who mingled with protesters.
The agents roughly bundled the protesters into brown prison vans in which they were driven to a nearby detention centre, Vyasna said.
Vyasna said in a statement on its website that the arrested included 17 journalists, although it said that most of the reporters were freed overnight.
The police have yet to give an official figure of the number arrested or how many people were freed overnight.
Protest rallies are rare in the tightly-policed former Soviet republic, but strikes organised through social networking sites have multiplied in recent weeks as the country faces a worsening economic crisis.
The country last week imposed price caps on all consumer goods to fight spiralling inflation.
Lukashenko, in power for 17 years, launched a crackdown on the opposition after mass protests following his landslide re-election victory in December.
The hardline tactics against demonstrators provoked international outrage, especially as the protesters had staged deliberately low-key "silent" rallies with no slogans or banners.
Speaking in neighbouring Lithuania last week, Hillary Clinton, the US secretary of state, said Belarus must "release political prisoners and embark on the path of democratic reform".
Dozens of opposition activists have already been imprisoned, including Lukashenko's leading rival in the elections, the former diplomat Andrei Sannikov who was jailed for five years.
In a speech on Sunday marking the anniversary of Minsk's liberation from occupation by Nazi Germany in 1944, Lukashenko warned his opponents not to dream of the "colour revolutions" that swept other ex-Soviet states in the past.
"They want to put us on our knees and reduce our independence to zero. This will not happen," he said.
The opposition had hoped to drown out Lukashenko's speech with their hand-clapping but failed to make any impact amid a massive security presence.
Ironically, no-one present, including those in the VIP stands, dared to applaud Lukashenko's speech for fear of being taken for a covert supporter of the opposition.
Opposition politician Stanislav Shushkevich, who led Belarus to independence in 1991 until he was ousted in 1994, was briefly detained by border police at the weekend as he travelled on a train from the Lithuanian capital Vilnius.