President Alexander Lukashenko of Belarus arrived in Russia to meet Vladimir Putin on Monday, seeking Moscow's backing after a fifth consecutive weekend of huge protests demanding his resignation.
Flight tracking data showed Lukashenko's plane touched down in the Black Sea resort of Sochi, where Putin often hosts visiting dignitaries.
The fate of the Belarusian leader is in Putin's hands as he faces the gravest crisis of his 26 years in power. Economic and military support from Moscow could help tip the balance in his favour as his security forces crack down hard on the opposition.
The Belarusian opposition accuses Lukashenko of rigging last month's presidential election, which he says he won fairly with 80% of the vote. Since then, thousands of people have been arrested and nearly all the opposition's key leaders have been detained, deported or forced to flee the country.
At least 100,000 protesters took to the streets of the capital Minsk on Sunday, taunting Lukashenko with chants of "You're a rat". Police said they detained 774 people at protests across the country.
Putin's actions so far suggest he has no desire to see the leader of a neighbouring ex-Soviet country toppled by pressure from the streets - even if Lukashenko has often proved a prickly and difficult ally.
The Kremlin leader said last month he had set up a "reserve police force" at Lukashenko's request, though it would be deployed only if needed. On Monday Russia will send paratroopers to Belarus for joint "Slavic brotherhood" military drills until Sept. 25, RIA news agency quoted the defence ministry as saying.
Russia has also offered to restructure Belarusian debt and support the banking system.
Belarus is the former Soviet republic with the closest political, social, economic and defence relationships with Russia. The two countries even proclaimed a "union state" in the 1990s, complete with a Soviet-style red flag.
The cost of further backing from Moscow could be Lukashenko's acceptance of even greater Russian dominance. The Kremlin has long pushed for closer integration, including a joint currency. Lukashenko has resisted some of those measures, and has had a difficult personal relationship with Putin.
Yet the Belarusian leader's position could become more precarious should the protests persist. Sunday's demonstration was one of the biggest yet.
"We have to show with this march that he doesn't control the country, that he is not in a position to speak on behalf of Belarusians," said Gennady, 35, a logistics worker who declined to give his last name.