Belarusian police on Sunday detained 125 protesters who had gathered in central Minsk, according to the Interior Ministry, as an unauthorized demonstration against President Alexander Lukashenko went ahead as planned.

Opposition sources estimated some 100,000 protesters marched through the Belarusian capital. Many of the participants carried placards mocking Lukashenko, who turned 66 on Sunday.

Participants also chanted "Happy birthday, you rat," and called for Lukashenko's arrest.

Russia's Vladimir Putin, however, congratulated Lukashenko in a phone call, in what could be seen as a sign of support for the Belarusian leader. The Kremlin said the two presidents agreed on Lukashenko visiting Moscow "in the coming weeks."

Videos shared on a Telegram-based news channel showed security forces as they began arresting people and pulling them into police vehicles as nearby protesters reportedly yelled, "shame!"

Police had cordoned off the central Independence Square in an attempt to keep demonstrators away from nearby government buildings.

DW's Nick Connolly shared images from the protest.

Read more: Belarus strips foreign journalists of accreditation on eve of rally

Third week of protests

On the last two Sundays, hundreds of thousands of people took to the streets in Belarus to protest against Lukashenko, who protesters call "Europe's last dictator."

The police response to the protests has fluctuated between extremely heavy-handed to mild since the rallies started earlier this month. This Sunday's arrests and others earlier this week could indicate that the government's tolerance toward the demonstrations is dwindling.

Read more: Putin says Russia ready to help with Belarus security

The country's Interior Ministry had warned its citizens from participating in the unauthorized rally, under threats of violence.

Lukashenko, who has ruled Belarus since 1994, recently claimed he'd won a landslide victory in the country's presidential vote. 

The leader of the opposition, Svetlana Tikhanovskaya, claimed she had won the election and called for the protests. She has since fled to neighboring Lithuania. 

The European Union does not recognize Lukashenko's victory, and is preparing sanctions against high-level Belarusian officials.  

kp,ls/dj (dpa, Reuters, Interfax)