Masked men detained Belarusian protest leader Maria Kolesnikova in central Minsk this morning and drove her away in a van and two of her allies also disappeared later, the opposition movement has said.
Ms Kolesnikova, a member of the opposition coordination council, is the last of three female politicians left in Belarus who joined forces before the 9 August presidential election to try to challenge veteran incumbent Alexander Lukashenko.
Before the election, Ms Kolesnikova had joined forces with opposition presidential candidate Svetlana Tikhanovskaya and with Veronika Tsepkalo, who has since left for Poland.
Opposition challenger Svetlana Tikhanovskaya, who claims she won the presidential election, left Belarus under pressure from authorities last month.
Ms Tikhanovskaya has taken shelter in neighbouring Lithuania, an EU member state.
A vocal critic of Mr Lukashenko, Ms Kolesnikova has played an important role in weeks of mass demonstrations and strikes by protesters who accuse Mr Lukashenko of rigging his re-election.
Mr Lukashenko, who has been in power for the last 26 years, denies that allegation and has accused foreign powers of trying to topple him in a revolution. He has responded with a crackdown which some of those detained say includes torture and beatings.
The EU's top diplomat has called on Belarus to immediately free the more than 600 people jailed following the crackdown on protests.
"The EU expects the Belarusian authorities to ensure the immediate release of all detained on political grounds before and after the falsified 9 August presidential elections," Josep Borrell said.
Facing his deepest crisis yet, Mr Lukashenko retains the support of Russian President Vladimir Putin, who has promised to send in police to support him if needed.
Ms Kolesnikova's abduction, if confirmed, comes as Belarusian authorities appear to be stepping up their efforts to try to halt the protests and obstruct the work of the opposition council, which they have accused of plotting to overthrow Mr Lukashenko.
Yesterday, tens of thousands of people demonstrated across the country demanding Mr Lukashenko step down. Security forces detained 633 protesters, Belarusian authorities said.
Police in Minsk were cited by Russia's Interfax news agency as saying they had not detained Ms Kolesnikova.
A witness, Anastasia, was cited by the Tut.By media outlet as saying she had seen Ms Kolesnikova pushed into a dark-coloured van by masked men in plain clothes in central Minsk.
She said Ms Kolesnikova's mobile phone had dropped to the ground during the tussle and that one of the masked men detaining her had picked it up before the van sped off.
The opposition council said two other activists, Anton Rodnenkov and Ivan Kravtsov, disappeared soon afterwards and said the authorities appeared to be systematically targeting its members.
"It's obvious that these methods are illegal and cannot lead to anything else apart from an escalation in the situation in the country, deepening the crisis and fuelling protests," it said in a statement.
"We see that the authorities have openly begun to use terror tactics in recent days," it added, calling the actions crimes against humanity.
Lithuanian Foreign Minister Linas Linkevicius likened what had happened to Ms Kolesnikova to something that the Stalin-era secret police in the Soviet Union would have done.
"Instead of talking to the people of Belarus, the outgoing leadership is trying cynically (to) eliminate (them) one by one," he wrote on Twitter.
"The kidnapping...is a disgrace. Stalinist NKVD methods are being applied in 21st century Europe. She must be released immediately."
Another leading activist, Olga Kovalkova, arrived in Poland on Saturday, saying she had been told she would face arrest if she stayed in Belarus.
The crisis is hitting the Belarus economy. Central bank figures published today showed the former Soviet republic had burned through nearly a sixth of its gold and foreign exchange reserves, or $1.4bn, in August, as it fought to prop up its rouble currency during the wave of unrest.