The United Nations and Western countries had told Moscow before the ruling to let Navalny go, and some countries have called for new sanctions on Moscow, which on Monday told them to mind their own business.
The ruling to remand him in custody for violating the terms of a suspended jail sentence, a day after he flew back to Russia for the first time since he was poisoned with a nerve agent last year, could be the prelude to him being jailed for years.
Moscow's prison service has applied to convert a suspended three-and-a-half-year embezzlement sentence in the same case, which he says was trumped up, into real jail time early next month. He faces three other separate criminal cases too.
As Navalny was led out of a police station, he told supporters that the only thing they needed to be afraid of was their own fear.
"Don't be afraid, take to the streets. Don't go out for me, go out for yourself and your future," Navalny said in a video posted to Twitter.
Supporters plan to rally across the country this Saturday, and an application for a 10,000-strong meeting at the end of this month has been lodged with Moscow authorities.
Over 70 Navalny supporters and journalists were detained across Russia on Monday, Monitoring group OVD-Info said.
Navalny (44) called his treatment illegal and accused President Vladimir Putin of throwing the criminal code out the window in fear.
The Kremlin did not respond, but has previously said Navalny must face justice if he has done anything wrong.
About 200 Navalny supporters had gathered outside the police station in freezing temperatures.
When they heard he had been remanded, they started chanting, "Disgrace!" and "Putin resign!"
Four masked police officers detained Navalny at passport control on Sunday evening as he returned to Russia after being treated in Germany for what German military tests showed was poisoning by a Novichok nerve agent, a version of events the Kremlin rejects.
The rouble weakened as investors weighed the risk of new sanctions against Moscow.
CALLS FOR SANCTIONS
Lithuania, Latvia and Estonia said they wanted European Union foreign ministers to discuss sanctions against Russia on Monday for detaining Navalny, whose foundation specialises in investigations into alleged official corruption.
Some of the foundation's targets have taken legal action and some critics have upbraided Navalny in the past for espousing what they say are overly nationalist views, something he rejects.
A Navalny ally released a list of eight Russian businessmen, bankers, government ministers and a state-backed journalist he said Navalny believed should be hit with Western sanctions.
Another possible target of any new penalties would be Nord Stream 2, a $US11.6 billion ($NZ16.2 billion) project to build a natural gas pipeline from Russia to Germany. Germany has supported the project, saying it is a commercial venture, and government spokesman Steffen Seibert said on Monday that had not changed.
The foreign ministers of Germany, Britain, France and Italy had earlier called for Navalny's release as did the UN human rights office. Jake Sullivan, one of US President-elect Joe Biden's top aides, and US Secretary of State Mike Pompeo have criticised the arrest.
The Russian Foreign Ministry brushed off the criticism.
"Respect international law, do not encroach on national legislation of sovereign states and address problems in your own country," Foreign Ministry spokeswoman Maria Zakharova wrote on Facebook.
Foreign Minister Sergei Lavrov said Western countries' expressions of outrage were designed to distract their citizens' attention from domestic problems and that Moscow was unfazed by potential damage to its image.
"We should probably think about our image, but we're not young ladies going to a ball," Lavrov told reporters.
Moscow residents interviewed by Reuters TV were divided on Navalny's detention.
"He probably did the right thing and acted like a real man (by returning)," said one Muscovite, Yuri Elizarov. "But from a political viewpoint he didn't, because nothing is probably going to change here in the coming years."
