Riot police and troops mounted a heavy presence in Minsk on Sunday and began detaining demonstrators as the Belarusian opposition launched another mass rally against the regime of strongman Alexander Lukashenko.

Protests are now into a third week since August 9 disputed elections in which Lukashenko claimed victory, while his opposition rival Svetlana Tikhanovskaya said she was the true winner after thousands flocked to campaign rallies. 

Two giant rallies on August 16 and 23 have seen some 100,000 protesters gather on Minsk streets in the largest demonstrations the country has ever seen. 

The Peace March started at 2pm local time (1100 GMT) on the central Independence Square. 

Riot police and police vans blocked off entrances to the square and put up metal barriers ahead of the rally, while numerous police vans were parked nearby. 

Columns of protesters began streaming towards the centre, carrying placards and the country's historic red-and-white flag, many with children in tow, as cars honked horns in support.

Local media posted video of trucks carrying water cannons driving towards the rally venue. 

Armed troops in balaclavas and without identifying badges also took up positions around a war memorial that has been a gathering point for marchers, blocking access. 

Police fenced off another popular meeting point, October Square, while a few hundred people had gathered there, an AFP journalist saw. 

The latest rally came amid a harsh crackdown on media freedoms. 

- 'Morally bankrupt' -
On Saturday the Belarusian foreign ministry withrew accreditation for numerous journalists working for international media, including AFP, the BBC and Radio Liberty / Radio Free Europe citing "counter-terrorism" grounds. 

The move was condemned by Germany and the United States. Tikhanovskaya, who has fled to the safety of Lithuania, on Saturday said that this step was "another sign that this regime is morally bankrupt" and resorting to "fear and intimidation." 

Lukashenko spoke to Russian President Vladimir Putin shortly before the protest began with the Kremlin leader wishing him a happy birthday. 

The Kremlin said they agreed to meet in Moscow "in the next weeks" and on their intentions to further strengthen Belarus-Russia's alliance," after Putin this week vowed military support for Lukashenko if needed. 

Putin said Russia had prepared a reserve of law enforcement officers to deploy if the situation got "out of control."

Reporters covering the protests have been detained and police have confiscated memory cards from photographers' cameras. 

The authorities have also shut off Internet access repeatedly, making it harder for independent media to report from the scene. 

Sunday's rally fell on Lukashenko's 66th birthday and Telegram messengers followed by the opposition urged people to bring flowers and "creative" handmade gifts reflecting their attitude to the authoritarian leader. 

On Saturday, around a thousand women marched in Minsk calling for new elections and prosecutions of police who used violence against demonstrators. 

"I'm afraid but I came for freedom and for us to have rule of law," said one participant, 32-year-old Yelena. 

On Sunday, more than 360 Belarusian sports figures including several Olympic athletes signed an open letter calling for new elections to be held according to international standards and condemning police violence. 

Lukashenko ordered brutal police tactics following the elections that led to the death of three men while hundreds were wounded. More than 7,000 people were detained.