More than 10,000 people were detained at recent rallies in Russia in support of Kremlin critic Alexei Navalny with many subjected to mistreatment in police custody, monitors said Wednesday.

Navalny's supporters took to the streets in cities across the country on consecutive weekends last month calling for his release from detention and denouncing Kremlin rule.

Hundreds more filled the streets of the capital Moscow Tuesday evening, after Navalny, 44, was ordered behind bars for nearly three years on old embezzlement charges.

At nationwide rallies over the last two weeks, more than 10,000 people were seized by police, the OVD-Info group that monitors opposition protests reported Wednesday.

Detainees are held for hours "in horrid and stuffy conditions, without food or the opportunity to use a bathroom," OVD-Info analyst Grigory Durnovo told Ekho Moskvy radio.

He added that lawyers from the group, which provides free legal aid to detained protesters, were at times not given access to detention centres.

Navalny called on Russians to take to the streets after he was detained last month on arrival in Moscow from Germany where he had been recovering from a poisoning with a Soviet-designed nerve agent.

A court in the capital on Tuesday ruled that he must spend two years and eight months behind in prison for violating the terms of a 2014 suspended sentence on fraud charges.

Russian police and investigators continuously issued warnings ahead of rallies, threatening arrests and criminal probes, to discourage protesters from joining.

Over the course of two weeks, several Navalny allies were also detained and key aides have been placed on house arrest pending criminal charges.

Navalny's arrest and the violent police crackdown on protesters has met with condemnation by international rights groups and Western governments, including the United States, Britain and France.