Belarusians chanting "Happy Birthday, you rat" and flying red-and-white opposition flags gathered near President Alexander Lukashenko's residence today as protesters kept up pressure on the veteran leader to resign.
They later dispersed peacefully.
The president, in office for 26 years, has shown no inclination to step down. For the second weekend in a row he appeared in a black cap and carrying an automatic rifle while walking around his residence, according to a photo published by Russia's RIA news agency.
Mr Lukashenko, who turned 66 today, is struggling to contain weeks of protests and strikes since winning a 9 August election his opponents say was rigged.
He denies electoral fraud and has said the protesters, whom he previously called "rats", are backed from abroad.
Russian President Vladimir Putin used a birthday phone call to invite Mr Lukashenko to visit Moscow, a sign of the Kremlin's willingness to back the president as he grapples with the unrest and the threat of new Western sanctions.
Tens of thousands of protesters streamed into central Minsk, carrying balloons, flowers and flags, earlier this afternoon. Belarus had a white-red-white flag for a brief period in the early 1990s and it has become a symbol of its anti-government protests.
Passing cars honked their horns in solidarity. Some women lay down in protest in front of a cordon of helmeted security forces.
Protesters then converged on President Lukashenko's residence, whichw as guarded by a wall of security forces carrying shields. A column of armoured military vehicles was seen driving towards the city centre.
Police made sporadic detentions throughout the day, bundling people into prisoner vans. At least 140 people were detained, the interior ministry said.
Some protesters resisted arrest by what appeared to be plain-clothes officers, an eyewitness said.
Earlier, video footage shared by local media showed women dressed in traditional dress laying several pumpkins in front of the main government building, a folk custom intended to signal the rejection of a suitor.
In a holiday atmosphere, the protesters occasionally sang songs. At one point, a man draped in a flag got down on one knee and made a marriage proposal to a woman who accepted with a hug and a kiss as bystanders cheered.
An aide to the president, Nikolai Latyshenok, ruled out holding talks with the opposition and said that, in his personal opinion, only around 20-30% of Belarusian society was against the president, Russian news agencies reported.