MOSCOW –  Three Russian opposition leaders were detained by police minutes after a demonstration to protest a government plan to raise the age for receiving state retirement pensions ended Sunday.

The demonstration in central Moscow had been authorized by city authorities and attracted thousands of people. No reason for the arrests was immediately given.

Oleg Stepanov, a top aide to Russian opposition leader Alexei Navalny, reported his detention on Twitter. He posted a photo of himself and the two others sitting in what appeared to be a police vehicle.

The Libertarian Party of Russia organized the demonstration. Kirll Samodurov, the party’s deputy chief, told the Interfax news agency that party chairman Sergei Boiko and rally leader Mikhail Chichkov were the other detainees.

The government’s proposal to raise the pension age for men from 60 to 65 and from 55 to 63 for women has sparked dissent across the political spectrum. The Communist Party sponsored a Moscow rally attended by an estimated 10,000 on Saturday.

The Bely Schetchik organization, which tracks attendance at public rallies, estimated the crowd at Sunday’s event at about 6,000.

Navalny, who is President Vladimir Putin’s most prominent foe, attended the demonstration, but did not address the crowd. He has been arrested multiple times for calling unauthorized protests.

The government says increasing the age for collecting pensions is necessary because of rising life expectancy rates, but opponents characterize it as robbery.

“This is not some hypothetical robbery. It’s not even corruption. It’s blatantly taking hundreds of thousands of rubles or even up to a million ($16,000) from some people,” Navalny said while standing in the crowd Sunday.

The average pension currently is about 14,000 rubles ($230) a month.