Demonstrators hit the streets of the capital before talks between Alexander Lukashenko and Russia's Vladimir Putin.
Belarus police violently detained hundreds of protesters as tens of thousands demonstrated in the capital Minsk in advance of talks between strongman Alexander Lukashenko and Russian President Vladimir Putin.
Masked policemen in uniform and plain clothes seized people who gathered for the "March of Heroes" demonstration on Sunday, pushing or punching them, video posted on the Belarusian news site Tut.by showed.
Oktyabrskaya Square in central Minsk was fenced off with barbed wire with armed law enforcement forces seen behind it. Independence Square was also sealed off.
Demonstrators were heading towards the Palace of Independence, President Lukashenko's residence.
"Soldiers rounded us up in several circles, people were selectively pulled out of the crowd and beaten," one unidentified demonstrator told Reuters news agency.
Lukashenko - in power for 26 years - is facing a groundswell of public anger after declaring a landslide win in last month's presidential election that his opponents say was rigged. He denies the allegations.
"I came out for freedom and I am going to protest until we win it through peaceful means," 60-year old marcher Oleg Zimin told AFP news agency.
He said he did not vote for Lukashenko last month. "He always lied to us," said Zimin.
The opposition urged Lukashenko not to "sell the country" ahead of his first face-to-face meeting with Putin since demonstrations began, which is set to take place in Russia on Monday.
A vast column protesters marched through the capital chanting "Long live Belarus" and "You're a rat," a taunt that has frequently been used against Lukashenko during demonstrations.
They came to a halt and chanted "fascists" as hundreds of riot police with shields blocked off the road.
Police said they had detained over 400 people in Minsk alone. A Reuters witness said that detentions were continuing on Sunday evening.
Al Jazeera's Step Vaessen, reporting by phone from Minsk, said the internet was blocked and security forces had been making it extremely hard for protesters to gather.
Yet, she said tens of thousands rallied in the centre of the capital, although at different locations than initially planned.
An Al Jazeera cameraman was briefly detained and nearly dragged into a van but escaped, Vaessen reported.
"Vans of masked policemen are driving around the city at high speed, stopping and snatching people from the street," she said. "It is very clear that the strategy today is to clamp down on any more moving towards the Sunday rally."
On Saturday, at least 5,000 people marched through the city demanding the release of a jailed opposition leader in the latest wave of mass protests after the August 9 presidential vote. Key opposition figures of Belarus have been either jailed or forced out of the country.
Vaessen said Lukashenko's meeting with Putin was crucial. "He wants to show that he has these protests under control, and images of very large gatherings are not something that he wants to see today."
She said the government and demonstrators were digging in and neither wants to compromise.
"It's a complete standoff. Lukashenko has repeated again and again that he is not willing to step down. People here are also not willing to stop the protests because they have started something they are calling the 'awakening of Belarus'. After so many years, 26 years of dictatorship, they have passed the point where they can accept it any more."
Peter Zalmayev from the Eurasia Democracy Initiative said the month-long mass rallies each Sunday are single-minded in their purpose, the removal of Lukashenko and his "iron-fisted rule".
"The one goal the protesters have is to get rid of the guy and they're doing it in a spectacularly democratic fashion with no single leader, with the leadership dispersed, and with a true popular uprising the likes of which Belarus has never witnessed," Zalmayev told Al Jazeera.
He said the only reason Lukashenko was "hanging on" was because of the loyalty of his security forces.
On Sunday, Russia's defence ministry said it would send paratroopers from its elite Pskov division to Belarus for the joint drills starting on Monday and running till September 25, the TASS state news agency reported.