Riot police in balaclavas harshly detained some 250 demonstrators as tens of thousands of people gathered for an opposition protest on the eve of talks between President Alexander Lukashenko and his main ally, Russia's Vladimir Putin.

Masked agents in uniform and plain clothes snatched people from the streets as they assembled for the “March of Heroes” demonstration, often pushing or punching them, a video posted on the Belarusian news site Tut.by showed.

“Some 250 people were detained in various districts of the capital,” the interior ministry said in a statement, adding that those detained were carrying flags and “offensive” placards.

Access to mobile internet was limited and central metro stations closed, with authorities moving police vans, military vehicles and barbed wire into the centre ahead of the protest.

Despite the violent detentions, tens of thousands of protesters gathered in the city centre for a new massive march, an AFP correspondent said.

'For freedom'

“I came out for freedom and I am going to protest until we win it through peaceful means,” Oleg Zimin, a 60-year-old protester, told AFP.

He said he did not vote for Lukashenko.

“He always lied to us,” he said.

Belarusians have been demonstrating against Lukashenko's disputed re-election for a month, with more than 100,000 people flooding the streets of Minsk for four straight weekends.

The new march came as Lukashenko's security forces have stepped up arrests of protesters and senior opposition figures who are still in Belarus.

The opposition announced the protest slogan “We won't let him sell the country” ahead of Lukashenko's first face-to-face meeting with Putin since demonstrations began, which is set to take place in Russia on Monday.

Analysts say Putin may seek to exploit Lukashenko's political vulnerability to wring concessions from him, but any agreements compromising Belarus's sovereignty and independence are likely to enrage Belarusian protesters further.

After a massive protest last Sunday, Maria Kolesnikova, one of three prominent women opposition figures, was jailed after she resisted expulsion and tore up her passport.

More than 600 people were detained last Sunday in one of the largest waves of arrests since the early days of the demonstrations.

On Saturday, masked riot police violently detained dozens of women demonstrators and threw them into vans during a smaller protest in Minsk.

'Heroic people'

Presidential candidate Svetlana Tikhanovskaya, who the protest movement says won the vote on August 9 but was forced out of the country, paid tribute to demonstrators ahead of the march.

“Over the past month we have become a truly heroic people,” Tikhanovskaya, a political unknown until the election, said in a video address.

“We are continuing our fight for freedom,” said the 38-year-old former stay-at-home mother, who was forced into exile in neighbouring EU member Lithuania.

The unprecedented protests broke out after Lukashenko, who has ruled the ex-Soviet state for 26 years, claimed re-election with 80 per cent.

Lukashenko has refused to step down and has turned to Russia for support to remain in power.

On Friday, the United States said it would impose new sanctions on Belarusian figures within days and warned Moscow that continuing to back the strongman would only alienate Belarusians.

Speaking to reporters, US Deputy Secretary of State Stephen Biegun asked how Moscow could “back such a regime and such violence against peaceful citizens". Historically Russians and Belarusians have enjoyed good relations and the opposition says the protests are not aimed at Russia.

Putin and Lukashenko are set to meet in the Black Sea resort of Sochi on Monday, with the Kremlin saying the talks will cover integration plans as well as key trade and energy projects.

Putin has been keen to unify Russia and Belarus, and Moscow has accompanied its offers of military and economic aid with calls for tighter integration.

Lukashenko has in the past ruled out outright unification with Russia but his options are now limited, analysts say.

His security forces have detained thousands of protesters, many of whom have accused police of beatings and torture. Several people have died in the crackdown.