Nearly 1,400 people were detained in a violent police crackdown on an opposition protest in Moscow, a Russian group that monitors police arrests said Sunday, adding that was 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 the detentions it logged for Saturday's protest reached 1,373 by early Sunday. The overwhelming majority of people were soon released but 150 remain in custody, OVD-Info and a lawyers' association providing legal aid to the detainees said Sunday.

Crackdowns on the anti-government protesters began days before the rally. Opposition leader Alexei Navalny was arrested and sentenced Wednesday to 30 days in jail for calling for Saturday's protest against election authorities who barred some opposition candidates from running in the Sept. 8 vote for Moscow city council.

Navalny was unexpectedly hospitalized Sunday with a severe allergy attack, his spokesperson said.

Kira Yarmysh said Navalny, who did not have any allergies beforehand, was taken from the Moscow jail to a hospital in the morning, arriving with severe facial swelling and red rashes. Hours later, she said Navalny was in a "satisfactory condition."

Potential candidates arrested twice

Russian police violently dispersed thousands of people who thronged the streets of Moscow on Saturday to criticize election authorities for disqualifying independent candidates from the vote .

Several protesters reported broken limbs and head injuries. Police justified their response by saying the rally was not sanctioned by authorities.

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 again in the evening.

Police officers detain a protester Saturday. (Maxim Zmeyev/AFP/Getty Images)

Police eventually cordoned off the City Hall and dispersed protesters from the area, but thousands of demonstrators reassembled in several locations nearby, where new arrests began. Police beat some of them to the ground with wide truncheon swings while other demonstrators tried to push police away.

Police said the protesters numbered about 3,500 people but aerial footage from several locations where people were rallying simultaneously suggests at least 8,000 protesters.

The U.S. Embassy in Moscow on Sunday decried the violent crackdown as "use of disproportionate police force." The Russian presidential human rights council said Sunday it was concerned about the police brutality.

Russian President Vladimir Putin stayed away from Moscow over the weekend. On Sunday, he led Russia's first major naval parade in years, going aboard one of the vessels in the Navy Day parade in St. Petersburg on the Gulf of Finland. The parade included 43 ships and submarines and 4,000 troops.