Alexander Lukashenko’s talks with Russian President Vladimir Putin in the Black Sea resort of Sochi came a day after an estimated 150,000 people flooded the streets of the Belarusian capital demanding his resignation.
The Interior Ministry said 774 people were arrested in Minsk and other cities in Belarus for holding unsanctioned rallies on Sunday.
Mr Putin said Russia will provide a £1.15bn loan to Belarus and fulfil all its obligations under a union treaty between the neighbours.
Speaking at the start of the talks, he emphasised the Belarusians themselves must settle the situation without any foreign meddling.
He commended Mr Lukashenko for his pledge to conduct a constitutional reform.
Protesters in Belarus have dismissed Mr Lukashenko’s re-election for a sixth term in the August 9 vote as rigged.
The United States and the European Union have criticised the election as neither free nor fair and urged the Belarusian leader to engage in talks with the opposition, a demand he rejected.
The opposition has dismissed Mr Lukashenko’s talk about constitutional reform as an attempt to win time and assuage the protesters’ anger.
Mr Putin hailed it as a “timely and reasonable” move that would help “reach a new level in the development of the political system”.
In a bid to win Moscow’s support, the 66-year-old former state farm director has tried to cast the protests as an effort by the West to isolate Russia, which sees the neighbour as a key bulwark against Nato and a major conduit for energy exports to Europe.
As he sat across the table from Mr Putin, Mr Lukashenko pointed at Nato’s drills near Belarus’ borders and said the two countries must strengthen their defence ties.
Mr Putin emphasised that Russian paratroopers who arrived in Belarus for joint drills that began on Monday will leave the country after the exercise.
Kremlin spokesman Dmitry Peskov told reporters after the four-hour talks that the leaders did not discuss the possibility of basing Russian forces in Belarus.
he also reiterated that Moscow regards Mr Lukashenko as Belarus’ legitimate president.
Russia and Belarus have a union treaty envisaging close political, economic and military ties but they have often engaged in acrimonious disputes.
Before the election, Mr Lukashenko repeatedly accused the Kremlin of pressing Belarus to abandon its independence.
But with the US and the EU criticising the election as neither free nor fair and readying a package of sanctions, Mr Lukashenko now has to rely squarely on Russia’s support.
Despite the frictions in the past, the Kremlin abhors the prospect of public protests forcing the resignation of the nation’s leader, fearing it could embolden Mr Putin’s critics at home.
Mr Putin quickly congratulated Mr Lukashenko on his re-election and promised to send Russian police to Belarus if protests there turn violent, noting that there is no need for that yet.
“We see Belarus as our closest ally and we will undoubtedly fulfil all our obligations,” the Russian leader told Mr Lukashenko during Monday’s talks.
I'm very sorry that you have opted to have a dialogue with the dictator and not the Belarusian people
Sviatlana Tsikhanouskaya, the main opposition challenger who moved to Lithuania a day after the vote under pressure from the authorities, warned Mr Putin any agreements he may reach with Mr Lukashenko will not stand.
“I’m very sorry that you have opted to have a dialogue with the dictator and not the Belarusian people,” she said on Monday.
“Any agreements signed with Lukashenko, who lacks legitimacy, will be retracted by the new government.”
Pavel Latushko, a former culture minister and ambassador to France who was forced to leave Belarus after joining the opposition’s Coordination Council, warned while the Kremlin is standing by Mr Lukashenko now it may move later to engineer his departure.
“Lukashenko discredits himself each day and when he completely loses his authority it would be easier for Moscow to replace him,” Mr Latushko said from Poland.
“The Kremlin already has made a decision and is moving to fulfil a careful plan to have Lukashenko removed.”
Alexander Klaskousky, an independent Minsk-based analyst, said the Kremlin realises a push for deeper integration between the two countries makes no sense now because of Mr Lukashenko’s precarious position.
Mr Klaskousky predicted the Kremlin might prod Mr Lukashenko to de-escalate the crackdown on protests while looking behind the scenes for a candidate to replace him.
“Massive protests aren’t abating and the barbed wire, water cannons and hundreds of detainees underline Lukashenko’s pitiful condition, forcing the Kremlin to start looking for an alternative,” Mr Klaskousky said.
“Putin would hardly want to put all eggs in one basket.”
As Belarusian authorities continue to target the opposition with pressure and arrests, the United Nations Human Rights Council agreed to hold an “urgent debate” on Belarus on Friday.
Western and Latin American countries supported the motion, while Venezuela and the Philippines sided with Belarus. African nations mostly abstained.
In a speech on Monday, UN High Commissioner for Human Rights Michelle Bachelet emphasised that all allegations of torture by the security forces should be documented and investigated.
German Chancellor Angela Merkel’s spokesman, Steffen Seibert, strongly urged the Belarusian authorities to stop using force against demonstrators, release all political prisoners and start a dialogue with the opposition.
“Unfortunately, every day brings new evidence that Lukashenko’s rule is supposed to be preserved with fear and repression,” he said.