Russian opposition leader Alexei Navalny, serving time in jail for calling for unauthorised protests, has been hospitalised after suffering an acute allergic reaction.
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.
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 1400 people in the Russian capital at the rally on Saturday in one of the biggest crackdowns in recent years against the opposition, drawing international criticism.
Navalny's spokeswoman Kira Yarmysh 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 he had never suffered from such reactions in the past.
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Spokeswoman for the US embassy in Moscow Andrea Kalan wrote on Twitter that the large number of detentions in Moscow and the "use of disproportionate police force undermine rights of citizens to participate in the democratic process".
The Polish Foreign Ministry in a statement called on Russian authorities "to stop using force against peaceful demonstrations and to refrain from arbitrary detentions".
In a separate incident on Sunday, Russian activist Dmitry Gudkov, who was among the opposition candidates barred from running in local elections, 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.
Biggest crackdown in a decade
Nearly 1400 people were detained in a violent police crackdown on an opposition protest in Moscow, the largest number of detentions at a rally in the Russian capital this decade.
OVD-Info, which has monitored the arrests since 2011, said the number of detentions it logged for Saturday's protest reached 1373 by early Sunday.
The overwhelming majority of people were soon released but 150 remain in custody, the group as well as a lawyers' association providing legal aid to the detainees said.
Russian police violently dispersed thousands of people who thronged the streets to protest election authorities for disqualifying independent candidates from a September 8 city council vote.
Several protesters reported broken limbs and head injuries. Police justified their response by saying the rally was not sanctioned.
Along with the arrests of the mostly young demonstrators, several opposition activists who wanted to run for the Moscow City Duma were arrested throughout the city before the protest.
They were released later in the day only to be re-arrested in the evening.
Police eventually cordoned off City Hall and dispersed demonstrators but thousands reassembled in several locations nearby where more arrests began.
Russian police beat some of them to the ground with wide truncheon swings while others tried to push officers away.
Police said the protesters numbered about 3500 but aerial footage from several locations suggested at least 8000.
The US Embassy in Moscow on Sunday decried the crackdown as "use of disproportionate police force". The Russian presidential human rights council said it was concerned about the police brutality.