A judge ordered Alexei Navalny to be remanded in custody for 30 days, the Russian opposition leader's spokeswoman Kira Yarmysh said on Twitter.
The ruling concluded an hours-long court hearing set up at a police precinct where the politician was held since his arrest at a Moscow airport.
Mr Navalny flew to Russia from Germany, where he had spent five months recovering from nerve agent poisoning that he blames on the Kremlin.
He was detained at passport control at Sheremetyevo airport after flying in Sunday evening from Berlin, where he was treated following the poisoning in August.
Mr Navalny's arrest prompted a wave of criticism from US and European officials, adding to existing tension between Russia and West.
German foreign minister Heiko Maas noted that Mr Navalny had returned of his own volition and said "it is completely incomprehensible that he was detained by Russian authorities immediately after his arrival".
"Russia is bound by its own constitution and by international commitments to the principle of the rule of law and the protection of civil rights," Mr Maas added.
"These principles must of course also be applied to Alexei Navalny.
"He should be released immediately."
The politician's allies said he was being held at a police precinct outside Moscow and has been refused access to his lawyer.
The court hearing into whether Mr Navalny should remain in custody was hastily set up at the precinct itself, and the politician's lawyers said they were notified minutes before.
"It is impossible what is happening over here," Mr Navalny said in video from the improvised court room, posted on his page in the messaging app Telegram.
"It is lawlessness of the highest degree."
Calls for Mr Navalny's immediate release have come from European Commission President Ursula von der Leyen, the office of the UN High Commissioner for Human Rights, UK Foreign Secretary Dominic Raab and top officials of other EU nations.
US President-elect Joe Biden's pick for national security adviser called on Russian authorities to free Mr Navalny.
"Mr Navalny should be immediately released, and the perpetrators of the outrageous attack on his life must be held accountable," Jake Sullivan tweeted.
The outgoing US secretary of state, Mike Pompeo, said the US "strongly condemns" the decision to arrest Mr Navalny.
Nevertheless, the judge ordered that Mr Navalny be remanded in custody until February 15, Ms Yarmysh said on Twitter.
Mr Navalny's lawyer Vadim Kobzev told the Interfax news agency that the defence plans to appeal against the ruling.
Mr Navalny's detention was widely expected because Russia's prisons service said he had violated probation terms from a suspended sentence on a 2014 money-laundering conviction.
The service said it would seek to have Mr Navalny serve his three-and-a-half-year sentence behind bars.
Russian foreign minister Sergey Lavrov said on Monday the stream of Western reactions to Navalny's arrest reflects an attempt "to divert attention from the crisis of the Western model of development".
"Navalny's case has received a foreign policy dimension artificially and without any foundation," Mr Lavrov said, arguing that his detention was a prerogative of Russian law enforcement agencies that explained their action.
"It's a matter of observing the law," he added.
Mr Navalny, 44, President Vladimir Putin's most prominent and determined foe, brushed off concerns about arrest as he boarded his flight in Berlin on Sunday.
"It's impossible. I'm an innocent man," he said.
Mr Navalny fell into a coma while aboard a domestic flight from Siberia to Moscow on August 20.
He was transferred from a hospital in Siberia to a Berlin hospital two days later.
Labs in Germany, France and Sweden, and tests by the Organisation for the Prohibition of Chemical Weapons, established that he was exposed to a Soviet-era Novichok nerve agent.
Russian authorities insisted that the doctors who treated Mr Navalny in Siberia before he was airlifted to Germany found no traces of poison.
Russia refused to open a full-fledged criminal inquiry, citing a lack of evidence that Mr Navalny was poisoned, and officials have challenged Germany to provide proof of the poisoning
Last month, Mr Navalny released the recording of a phone call he said he made to a man he alleged was a member of a group of officers of the Federal Security Service, or FSB, who purportedly poisoned him in August and then tried to cover it up.
The FSB dismissed the recording as fake.
Mr Navalny has been a thorn in the Kremlin's side for a decade, unusually durable in an opposition movement often demoralised by repression.
Russian authorities have launched multiple criminal investigations against him, and he has been tried and convicted in two separate criminal cases widely seen as politically motivated.
In December 2014, Mr Navalny was convicted on charges of fraud and money-laundering and received a 31/2-year suspended sentence, which he denounced as politically motivated and the European Court of Human Rights found "arbitrary and manifestly unreasonable" three years later.
The sentence carried a probation period that was due to expire in December 2020.
Authorities said the politician was subject to regular in-person checks with law enforcement officers as one of the conditions of his probation.
Russia's prison service first accused Mr Navalny of not appearing for these checks on December 28 - two days before the probation period was supposed to end.
Mr Navalny and his team rejected the accusations and said the move was an attempt by the Kremlin to keep the politician from coming back to Russia.
Three days before his return to Moscow, the prison service alleged in a statement that Mr Navalny repeatedly failed to appear for the checks, including when he was convalescing in Germany, and said it was "obligated to undertake actions to detain" the politician.
Mr Navalny had repeatedly said he would come back to Russia despite threats of arrest, saying he did not leave the country by choice, but rather "ended up in Germany in an intensive care box", and said he was still dedicated to his cause.
"I will go back to Russia, I will continue my work. No other possibility has ever been considered or is being considered," Mr Navalny said in October.
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