About 700 people had tried to march along Minsk's main avenue, but were blocked by a cordon of riot police wielding clubs and holding shields. After a standoff, arrests began.
"They're beating the participants, dragging women by the hair to buses. I was able to run to a nearby courtyard," demonstrator Alexander Ponomarev said.
There were no immediate figures on how many people were taken into custody.
Earlier, police raided the offices of a prominent rights group, detaining dozens of people on the day of a planned protest, including foreign rights workers.
Police also detained dozens in the streets and seized leading opposition leader Vladimir Nekliayev as he was returning from Poland, taking him off the train at the border and placing him in a detention facility.
Viasna, a non-governmental organization (NGO) that had been tracking arrests and protest rallies in Belarus in recent weeks, said riot police had broken down the door, "put people face down on the floor and told them to stay there".
"There were 57 people detained, including foreign observers," it said on its website.
Those detained were taken to a police station, where they were told they are "suspected of banditism," searched and let out of the station in small groups after most of the protest had been broken up, the group's lawyer Anastasiya Loiko said.
Belarus has seen an unusually persistent wave of protests over the past two months against President Alexander Lukashenko, who has ruled since 1994. After tolerating the initial protests, authorities cracked down. Lukashenko this week alleged that a "fifth column" of foreign-supported agitators was trying to bring him down.
In his 23 years as president, Lukashenko has stifled dissent and free media and retained much of the Soviet-style command economy.
Authorities late Friday told organizers that the event would be illegal. On Saturday, scores of armored police trucks and water cannons, as well as officers armed with automatic rifles could be seen in the city.
‘Heavy police presence’
The square where the protest had been set to start was blocked by heavy police presence, with the metro exits sealed.
Police detained people at the scene, putting them in vans, but several hundred managed to walk with Belarusian red-and-white flags shouting "Shame!" before being broken up as riot police lined up to block main streets brandishing their shields.
Several journalists were also detained in Minsk and in Gomel, a city in southeastern Belarus, according to the Belarus Association of Journalists NGO. The team from Belsat, an opposition channel based in Poland, had their camera smashed, it said.
The Amnesty International rights group said on its Russian-language Twitter account that dozens of people were grabbed off the street "indiscriminately".
Dozens had already been arrested ahead of Saturday's event, as state television aired reports of alleged weapons caches discovered while police armed with automatic rifles were in the city center for the first time in decades.
Many had planned to travel to the capital from the provinces for the protest. Belarusian railway monopoly halted online sales for several hours overnight Friday to Saturday, ostensibly due to "technical works."
