Russian President Vladimir Putin used a birthday phone call on Sunday to invite Belarusian leader Alexander Lukashenko to visit Moscow, a sign of support from the Kremlin as thousands of protesters streamed into central Minsk demanding that Lukashenko step down.

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

"It was agreed to hold a meeting in Moscow in the coming weeks," the Kremlin said in a statement after the leaders' call.

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

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.

Thousands of people, many waving red-and-white opposition flags and chanting "long live Belarus," marched toward Minsk's Independence Square, which was cordoned off by police in anticipation of the protest.

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