At least 100,000 Belarusians took to the streets in the capital of Minsk on Sunday in one of the biggest protests yet against President Alexander Lukashenko after he claimed victory in a disputed election last month that his opponents say was rigged.
Police said they detained more than 400 protesters in Minsk, with arrests continuing into the evening.
With public outrage building against Lukashenko, who has ruled the former Soviet bloc nation for 26 years, Russia said it would support him by sending paratroopers to Belarus for "Slavic Brotherhood" joint drills.
Protesters claim the August 9 presidential election was won by Sviatlana Tsikhanouskaya. Key opposition leaders have since either been jailed or fled the country, with Tsikhanouskaya now in Lithuania.
Lukashenko has rejected claims the election was rigged and contends that foreign powers are behind the protests.
Throngs of protesters marched through Minsk toward a government district Sunday, chanting "Long live Belarus" and "You're a rat," a common taunt against Lukashenko.
Coming to a halt, they chanted "fascists" as hundreds of riot police with shields blocked a road.
The Interfax Russian news agency reported that shots were fired into the air to keep protesters away from an area of Minsk where the Belarusian leadership lives.
The unrest came as Lukashenko prepared to travel to Russia on Monday for talks with President Vladimir Putin.
Moscow has expressed support for Belarus, potentially restructuring its debt and offering to send in Russian riot police if needed.