A leading member of the Belarus opposition council, Maxim Znak, has gone on hunger strike in jail after being charged with seeking to harm national security.
The lawyer is among seven leaders of the Co-ordination Council, five of whom are either in jail or in exile.
In Geneva the UN Human Rights Council voted to launch closer UN monitoring of abuses in Belarus.
Delegates deplored President Alexander Lukashenko’s post-election crackdown.
For five Sundays running more than 100,000 protesters have filled the streets of the capital Minsk, demanding that the long-time president step down. His 9 August landslide victory has been widely condemned as rigged.
Mr Znak’s lawyer, quoted by the independent Tut.by website, said the activist’s hunger strike was aimed at exposing the “absurdity of prosecuting people for freely expressing their opinions”.
Mr Znak felt he had no other way to fight against injustice and arbitrary actions, while in detention, the lawyer said.
Earlier on Friday another leading council member, Sergei Dylevsky, was released from jail, having spent 25 days in custody.
He and Nobel literature laureate Svetlana Alexievich are the only Co-ordination Council leaders still in Belarus and not behind bars, the Belapan news agency reports.
Belarus prosecutors accuse the Co-ordination Council of trying to stage a coup.
Via videolink from Lithuania the main opposition leader, Svetlana Tikhanovskaya, addressed the UN council’s “urgent debate” – only the sixth such debate in its 14-year history.
She said human rights violations by the Belarusian authorities meant the international community had “a right to react in [the] strongest terms”.
“The scope and the brutality of the extensive force used by the regime is in clear violation of all international norms,” she said.
The UN council’s vote for closer UN human rights monitoring of Belarus came despite Russian attempts to water the resolution down and interruptions of Ms Tikhanovskaya’s address by the Belarus Ambassador Yuri Ambrazevich.
The resolution, drafted by the EU, calls for a UN report with recommendations by the year’s end on the human rights situation in Belarus.
UN Special Rapporteur Anaïs Marin told the session that more than 10,000 peaceful protesters had been “abusively arrested” and “over 500 cases of torture, committed by state agents, have been reported to us”.
“I have been informed of allegations of rape, electrocution, and other forms of physical and psychological torture,” she said.
The Belarus foreign ministry described the resolution as a “dangerous precedent” and accused the UN council of interfering in the country’s internal affairs.