Riot police fired warning shots into the air, used stun grenades and arrested more than 200 people to deter tens of thousands of Belarusians who marched through Minsk on Sunday to demand veteran leader Alexander Lukashenko leave power.

Mass demonstrations have flooded the capital for 12 straight weeks since a disputed election, ratcheting up pressure on the embattled leader of 26 years who rejects accusations the vote was rigged and says he has no intention of quitting.

This week Lukashenko partially closed the border to the west, replaced his interior minister and said that any protester who lays a hand on officers policing the protests should "at least leave without hands."

Tens of thousands of people swept through Minsk in at least two columns, the Nasha Niva newspaper reported. The Vesna-96 rights group published the names of 221 people who had been detained.

Senior police officials have threatened to use firearms against protesters if needed.

"The situation is really alarming and everyone's mood is tense," one protester who requested anonymity said by phone."

Police buses and equipment are constantly driving past the column. It feels like people are ready for any kind of escalation.

"A witness told Reuters that riot police used force to disperse marchers who had marched towards Kurapaty, a site on the outskirts of Minsk that is a memorial to victims of execution by Soviet secret police."

People got to a field near Kurapaty, (police) buses pulled up and chased after people at top speed, then they started throwing grenades.

What's more, they were throwing them into the thick of the crowd," the witness said by phone on condition of anonymity.

A man in civilian clothing chased a resident through a courtyard near one protest route firing a paintball gun at them in footage published by the RFE/RL media outlet.

Video posted on opposition social media showed a crowd of people chanting "We believe, we can, we will win!" while marching through the streets. The video footage could not independently be verified.

Mobile phone internet in the capital was unavailable and several metro stations were briefly closed down.