Belarus authorities raided the offices of a prominent rights group on Saturday, detaining dozens of people, including foreign rights workers, ahead of a planned protest by opponents of President Alexander Lukashenko.

The police also detained dozens in the streets and seized a 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.

Scores of people that turned up for the 2.00pm local time rally were grabbed by riot police and placed in vans, including several journalists. Some were beaten, an AFP correspondent observed.

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Viasna, a nongovernmental organisation that had been tracking arrests and protest rallies across Belarus in recent weeks, said riot police had broke 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.

Viasna's director, Ales Beliatski, later told AFP that about 1,000 people had been detained on Saturday.

The people seized at Viasna's offices 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.

"They put us on the ground, and they took some telephones," Masha Chichtchenkova, a Franco-Belarusian member of Front Line Defenders, told AFP after her release.

"They put us in a minibus and took us to a gymnasium," she said.

Saturday's protest was the latest in a series of events against Lukashenko's authoritarian regime, and the largest since the mass demonstrations that followed his disputed re-election in December 2010.

Thousands have attended rallies in recent weeks to oppose a controversial new tax on "spongers" or "freeloaders" - those who work less than six months a year - as the country suffers an economic slump, with the swell in protests alarming the government.

AP

Authorities late Friday told organisers that the event would be illegal. On Saturday, scores of armoured police trucks and water cannons, as well as officers with automatic rifles, were deployed in the city.

The European Commission called for all detained protesters to be "immediately released" by the authorities.

"Respect for the fundamental rights and freedoms, including of expression, association and assembly, needs to be upheld," the commission's delegation to Belarus said in a statement.

State media silence

Dozens had already been arrested in the days 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 centre for the first time in decades.

But there was no mention of the harsh crackdown by state media outlets on Saturday evening.

The police had blocked off the Minsk square where the protest was to start and sealed off metro exits, and hauling off several people at the scene into vans.

But several hundred others managed to walk with Belarusian red-and-white flags shouting "Shame!" before being broken up as riot police brandishing shields lined up to block main streets.

Several journalists were also detained in Minsk and in Gomel, a city in southeastern Belarus, according to the Belarus Association of Journalists. The team from Belsat, an opposition channel based in Poland, had their camera smashed, it said.

EPA

Amnesty International said on its Russian-language Twitter account that dozens of people were grabbed off the street "indiscriminately".

Nekliayev, the opposition leader who was set to speak at the protest, was stopped at the border Saturday morning on his way to Minsk, his wife told AFP.

"He is in a detention facility in Brest," Olga Nekliayeva said, referring to the city in southwestern Belarus close to the Polish border.

Many had planned to travel to the capital from the provinces for the protest, but the Belarusian railway monopoly halted online sales for several hours overnight Friday to Saturday, ostensibly for "technical works".