Navalny, a prominent critic of President Vladimir Putin, was remanded in custody for 30 days on Jan. 18 after flying back to Russia for the first time since being poisoned with a military-grade nerve agent in August.
A court at the time ordered him detained for alleged parole violations, which he denied. With various legal cases pending against him, Navalny, 44, could face years in jail. The West has called on Russia to release him and tens of thousands of Russians protested against his jailing on Saturday.
Addressing the presiding judge by video link from jail before the ruling, Navalny demanded to be released and railed against what he said were absurd allegations he said had been trumped up by Russia's authorities to sideline him for political reasons.
"We'll never allow ... these people to seize and steal our country. Yes, brute force is on your side now. You can...put me in handcuffs. (But) that will not continue forever," he said.
His lawyers said Navalny would appeal against the ruling to keep him in custody, the Interfax news agency reported.
After the ruling was handed down, Navalny said to the judge: "Everything was clear to me before the start of the court hearing, thank you."Navalny's allies have called for new protests this weekend to demand his release. The authorities have said any demonstrations will be illegal and broken up.