Belarusian security police searched offices and homes of lawyers and human rights activists today detaining at least 10 people.
It comes as part of a new crackdown against opponents of President Alexander Lukashenko, several rights groups and media said.
The country's notorious KGB has moved to shut down non-state media outlets and human right groups after mass protests last August against a presidential election the opposition said was rigged.
Police searched offices of at least 14 rights groups, media, NGOs and charity groups, including the human rights organisation, Viasna-96, and the country's oldest political party, Belarusian People Front, the groups said.
Sviatlana Tsikhanouskaya, the protest leader who fled Belarus last year and is currently visiting Ireland, said Mr Lukashenko's regime was taking revenge on human rights activists and media.
"The (regime) accomplices have already threatened civil society in response to sanctions - they hope they can feel his power again if everyone in the country is silent," she wrote on Telegram messenger.
Ms Tsikhanouskaya said at least 10 people were detained.
Thomas Byrne, Minister for European Affairs, said he strongly condemned the "wide-scale detentions of human rights defenders, activists and journalists in Belarus today."
In a post on Twitter, he continued: "I will be discussing this disturbing further escalation with Sviatlana Tsikhanouskaya tomorrow."
Viasna-96 said it was unable to contact its leader, Ales Byalyatski. It is the largest rights body in the former Soviet country and is the main source of information on political detentions and arrests.
As of yesterday evening, 555 people had been recognised as political prisoners in Belarus, it said.
Belarus security officials were unavailable for comment.
The independent Imena media and charity group, which does not consider itself an opposition outlet, also said that police had searched its offices.
Nasha Niva newspaper reported that police were trying to search the offices of another rights group, Belarusian Helsinki Committee.
Authorities in Belarus said last week they had blocked Nasha Niva's website. They later said they had detained the editor and three journalists.
Today's raids come a day after President Lukashenko, who has been hit with a slew of sanctions by Western nations, met with Russian leader and key ally Vladimir Putin in Saint Petersburg.
The head of the Belarusian Helsinki Committee Oleg Gulak wrote on Facebook that the group's "office door is being broken down".
Two opposition groups and the Belarusian Association of Journalists - which won an international media award last year - also reported raids.
The head of an organisation campaigning to abolish the death penalty, Andrei Poluda, was detained following a raid at his home.
In Minsk, raids also took place at an organisation promoting gender equality in the workplace, a social charity, an independent trade union and an economic research centre.
The home of prominent independent economist Yaroslav Romanchuk was also raided.
