(Reuters) - Tens of thousands of Belarusians chanting “Happy Birthday, you rat” and flying red-and-white opposition flags gathered near President Alexander Lukashenko’s residence on Sunday as protesters kept up pressure on the veteran leader to resign.

Lukashenko, who turned 66 on Sunday, is struggling to contain weeks of protests and strikes since winning an Aug. 9 election his opponents say was rigged. He denies electoral fraud and has said the protests are backed from abroad.

Russian President Vladimir Putin used a birthday phone call to invite Lukashenko to visit Moscow, a sign of the Kremlin’s willingness to back Lukashenko as he grapples with the unrest and the threat of new Western sanctions.

Protesters streamed into central Minsk, carrying balloons, flowers and flags, in the 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 Lukashenko’s residence, which was guarded by security forces carrying shields, and water canon and prisoner vans. A column of armoured military vehicles was seen driving towards the city centre, Russia’s Interfax news agency reported.

Police made sporadic detentions throughout the day, bundling people into prisoner vans. At least 125 people were detained, Russia’s RIA news agency quoted the interior ministry as saying. Some protesters resisted arrest by what appeared to be plain-clothes officers, a witness said.

Earlier on Sunday, 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.

One of the country’s largest mobile operators, A1, said it had reduced the capacity of mobile internet bandwidth at the government’s request.

Belarus is Russia’s closest ex-Soviet ally and its territory is an integral part of Moscow’s European defence strategy. Nevertheless, Lukashenko is seen in Moscow as a prickly partner.

In the biggest sign yet of Russia’s willingness to intervene to prop up Lukashenko, Putin said on Thursday the Kremlin had set up a “reserve police force” at Lukashenko’s request, although it would be deployed only if necessary.

“It was agreed to hold a meeting in Moscow in the coming weeks,” the Kremlin said in a statement after the leaders’ call on Sunday.

The European Union is gearing up to impose new sanctions on Belarus. Lukashenko, in office for 26 years, threatened on Friday to cut off European transit routes across his country in retaliation.