The U.S. State Department called for the immediate release of Navalny and condemned the Russian government’s crackdown on opposition figures and independent media.
During his time behind bars, his social media accounts have been regularly updated with posts about life in prison.
"Foreign agents" are subject to numerous constraints and laborious procedures, or they face heavy penalties. They must indicate this status in all their publications.
By the time Navalny’s sentence was extended for another nine years by a court on Tuesday, not much dissent could be mustered. The Kremlin had worked hard to see to that.
Navalny, President Vladimir Putin's most vocal domestic critic, was jailed last year on old fraud charges after surviving a poison attack with Novichok nerve agent that he blames on the Kremlin.
Navalny calls on Russians to take to the streets against invasion of Ukraine by an 'obviously insane czar'
The Putin critic who survived poisoning with a nerve agent and who has millions of followers on social media declared, "we will fill prisons and paddy wagons with ourselves."
Top opposition leader Alexei Navalny, his ally Nikolai Lyaskin and a number of their supporters were detained in Moscow as police roughly rounded up protesters.
Alexei Navalny was on Saturday detained at an unsanctioned rally in central Moscow, two days ahead of Vladimir Putin's inauguration for a fourth Kremlin term.
Navalny, who was barred from challenging Putin in the March presidential election, had called on Russians to stage a day of rallies across the country ahead of Putin's inauguration.
Facing competitors such as a former reality TV star and a director of a fruit farm, Putin is all but guaranteed to win the vote and extend his Kremlin stint to 2024 with a fourth term in office.
Putin is predicted to win with around 70 percent of the vote despite a lacklustre campaign and his refusal to participate in televised debates.
On Sunday, police broke into Navalny's headquarters in the Russian capital using a saw, to interrupt a live broadcast covering a series of protests in the east of the country.
Navalny called on Russians to defy authorities and stage rallies calling for an active boycott of March 18 presidential elections despite the prospect of mass arrests.
Navalny, who has declared his intention to stand for president in 2018, was released in a secret location in Moscow early Sunday to evade media attention.