Unidentified people have detained Belarusian protest leader Maria Kolesnikova in central Minsk and drove her off in a minivan, the Belarusian Tut.By media outlet cited a witness as saying.
Kolesnikova is the last of three female politicians left inside Belarus who joined forces before an August 9 presidential election to try to challenge veteran incumbent Alexander Lukashenko.
A vocal critic of Lukashenko, she has played an important role in the country's post-election political crisis which has seen weeks of mass protests and strikes by people who accuse Lukashenko of rigging his own re-election, something he denies.
Her abduction, if confirmed, comes as Belarusian authorities appear to be stepping up their efforts to try to break protesters' momentum.
Tens of thousands of people demonstrated on Sunday and security forces detained 633 protesters, Belarusian authorities said.
Kolesnikova's allies said they were checking the report of her detention. Police in Minsk were cited by Russia's Interfax news agency as saying they had not detained her.
Before the election, Kolesnikova had joined forces with opposition presidential candidate Sviatlana Tsikhanouskaya who later fled to Lithuania, and with Veronika Tsepkalo, who has also since left the country.
Another leading activist, Olga Kovalkova, arrived in Poland on Saturday, saying she had been told she would face arrest if she stayed in Belarus.
Earlier on Monday, central bank figures showed Belarus had burned through nearly a sixth of its gold and foreign exchange reserves, or $US1.4 billion, in August, as it fought to prop up its rouble currency during the wave of unrest.
Kolesnikova had announced on August 31 that she was forming a new political party, Together, with the team of jailed opposition figure Viktor Babariko with whom she had previously worked.
The European Union, meanwhile, aims to impose economic sanctions on 31 senior Belarus officials including the country's interior minister by mid-September, three EU diplomats said, in response to the election the West says was rigged.
Names could still be added or taken off the list, but the diplomats said formal agreement is likely to come on September 21, when EU foreign ministers hold their next scheduled meeting.
Lukashenko will not be sanctioned. Germany, who holds the six-month presidency of the EU, wants more time for dialogue and to leave open the possibility of adding the president's name at a later stage.