Belarus's embattled leader, Alexander Lukashenko, has arrived in Russia's Black Sea resort of Sochi for talks with Russian President Vladimir Putin.
It will be their first face-to-face meeting since protests escalated in Belarus last month, following the disputed presidential election.
Belarusian opposition leader Svetlana Tikhanovskaya said she regretted Mr Putin's "dialogue with a dictator".
Mr Putin recognises Mr Lukashenko as the legitimate president of Belarus.
He says he has a Russian police force ready to intervene if the protests get out of control.
There is speculation he wants closer ties in exchange for continued support.
Kremlin spokesman Dmitry Peskov said oil and gas co-operation, state debts and other economic ties in the Belarus-Russia "union state" would be discussed, as well as some international issues, but no joint documents would be signed, nor would there be a press conference.
The two countries are traditionally very close, though Mr Lukashenko has resisted Russian pressure to deepen their union.
A joint military exercise is starting near the western Belarusian city of Brest.
On Sunday, at least 100,000 people marched through the Belarusian capital Minsk and other cities to demand that Mr Lukashenko resign. Police said they detained about 400 people.
It was the fifth Sunday since the election that such crowds have thronged the capital.
Ms Tikhanovskaya, living in Lithuania after being forced into exile, said on her Telegram channel: "I want to remind Vladimir Putin, whatever you agree on in Sochi will not have legal force.
"Any agreements signed with illegitimate Lukashenko will be revisited by a new government because the Belarusian people no longer trust Lukashenko and did not support him in the elections. I regret that you have decided on dialogue with a dictator and not with the people of Belarus."
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The Belarusian opposition accuses Mr Lukashenko, who has been in power for 26 years, of rigging the presidential election. Western governments agree that the election was tainted by many irregularities.
Mr Lukashenko has insisted that he won fairly with 80% of the vote and has painted the protests as Western-backed and anti-Russian.
His opponents, however, say their dispute is with him, and insist they are not anti-Russian.