Belarus said on Monday that police had detained more than 600 people at weekend protests and the opposition said a senior figure had been snatched off the streets, as authorities intensified efforts to end weeks of demonstrations.

The opposition's Coordination Council said one of its high-profile members Maria Kolesnikova had been "kidnapped by unknown people in central Minsk" along with a spokesman and executive secretary.

"Their whereabouts are unknown," it said, accusing President Alexander Lukashenko's regime of "openly using methods of terror".

Lukashenko's main rival Svetlana Tikhanovskaya said the abductions were an attempt to disrupt the work of the Coordination Council.

It was set up to ensure a peaceful transfer of power after opposition candidate Tikhanovskaya rejected Lukashenko's claim to have won an August 9 presidential election with 80 per cent of the vote.

"The more they try to scare us, the more people will take to the streets," Tikhanovskaya said in a statement.

The Coordination Council demanded the detainees' immediate release and the free return of other activists who have left the country under official pressure.

A spokesman for the European Commission, Peter Stano, said the EU's executive branch was trying to establish what happened to those missing and condemned the Belarusian authorities' actions as "unacceptable".

EU sanctions 'very soon' 

He said the EU hoped to impose sanctions "very soon" after drawing up a list of individuals responsible for police violence against protesters and rigged elections.

The disputed election has sparked demonstrations that have seen tens of thousands take the streets of the ex-Soviet country of 9.5 million on Russia's western borders, in an unprecedented challenge to Lukashenko's 26 years in power.

An AFP journalist said the crowd of demonstrators waving the opposition’s red-and-white flag on Sunday appeared to be as large or larger than on the previous three Sundays, when more than 100,000 people rallied in the streets of Minsk.

But police also appeared to be stepping up a campaign to quash the demonstrations, with troops, water cannon and armoured vehicles deployed in the city centre on Sunday.

Local media reported hooded men in civilian clothes with batons chased and beat demonstrators.

Interior ministry spokeswoman, Olga Chemodanova, said on Monday that 633 people had been detained on Sunday for illegal mass gatherings, one of the largest wave of arrests since the early days of the demonstrations.

Security forces cracked down hard on the first demonstrations, with thousands arrested and many of those detained accusing police of beatings and torture, but then eased off after a widespread domestic outcry and international condemnation.

Kolesnikova’s office said witnesses described her being snatched off the street in the capital Minsk on Monday morning by unidentified men in black who bundled her into a minibus marked "Communications”.

Kolesnikova’s office said she was unreachable and her phone was switched off.

The Coordination Council said its press secretary Anton Rodnenkov and executive secretary Ivan Kravtsov had also disappeared, while police said they had no information on detentions.

Belarusian authorities had already detained several members of the Coordination Council. One, Olga Kovalkova, said on Saturday she was in Poland after security services threatened her and took her to the border.

Kolesnikova and other members including Nobel Literature Prize winner Svetlana Alexievich have faced questioning in a probe over an alleged bid to seize power.

Kolesnikova, 38, is the only one of the trio of women who fronted Tikhanovskaya’s campaign to remain in Belarus.

Tikhanovskaya has taken shelter in neighbouring Lithuania and her other campaign partner, Veronika Tsepkalo, is now in Ukraine.

Kolesnikova, a trained flautist and music teacher, entered politics to run the campaign of another opposition politician, ex-banker Viktor Babaryko, who attempted to stand for president against Lukashenko but was jailed and barred from running.