Navalny was jailed this week for 30 days for calling for an unauthorised march to protest against the exclusion of several opposition candidates from a local election later this year.
Russian opposition leader Alexei Navalny, currently serving time in jail for calling for unauthorised protests, was hospitalised after suffering an acute allergic reaction on Sunday, his spokeswoman said.
Authorities say the opposition candidates were barred because they failed to collect enough genuine signatures backing them, an allegation they reject as false.
Police rounded up more than 1,000 people in the Russian capital at the march on Saturday in one of the biggest crackdowns in recent years against the opposition.
Kira Yarmysh, Navalny's spokeswoman, wrote on Twitter that he had been hospitalised on Sunday morning with "severe swelling of the face and skin redness."
She said the cause of Navalny's allergic reaction was unknown and that he had never had suffered from such reactions in the past.
The Moscow hospital where Navalny's spokeswoman said he was being treated could not immediately be reached for comment.
In a separate incident on Sunday, Russian activist Dmitry Gudkov, who was among the opposition candidates barred from running in local elections this year, said he was detained and taken to a Moscow police station.
The reason for Gudkov's detention was not immediately clear, his spokesman Alexei Obukhov told Reuters.
Russia's interior ministry did not respond to a request for comment on Navalny and Gudkov's detention.
Obukhov said Gudkov, a former MP who challenged pro-Kremlin initiatives, had been detained as he walked out of a shop near his home on Sunday, where he had been buying food for the protesters still being held.
Navalny, Russia's most prominent opposition figure, has served several stints in jail in recent years for organising anti-government demonstrations.
The European Court of Human Rights last year ruled that Russia's arrests and detention of Navalny in 2012 and 2014 were politically-motivated and breached his human rights, a ruling Moscow called questionable.