Several leading Nordic and Baltic conservative politicians joined an online protest calling for the release of Russian opposition leader Alexei Navalny and his supporters.
"We are concerned by mass detentions of peaceful demonstrators in Moscow and other Russian cities. They should be released without delay," Erna Solberg, Norway's prime minister, said on Tuesday.
The online protest - staged on Facebook - was organized by Ulf Kristersson, leader of Sweden's opposition conservative Moderate Party.
Similar calls were made by Carl Bildt, former foreign minister and prime minister of Sweden; Alexander Stubb, a former Finnish prime minister and foreign minister, as well as Lithuania's Foreign Minister Gabrielius Landsbergis.
The crackdown on government opponents showed that Russian leader Vladimir Putin has "been learning" from Belarus' long-serving leader Alexander Lukashenko, said Sandra Kalniete, Latvia's former foreign minister and member of the European Parliament.
Use of force
She also criticized the "disproportionate use of force" against protesters in Russia.
A week ago, Finnish Prime Minister Sanna Marin also demanded from Russia that Navalny should be released "without delay."
According to activists in Russia, about 3,500 people were detained at the weekend in some of the biggest protests seen under Putin.
Navalny returned to Russia on 17 January, after months in Germany where he received treatment after he was poisoned with the nerve agent Novichok. He has blamed Russian authorities for the poisoning.
Navalny was last week sentenced to 30 days of pretrial detention on charges of violating the terms of his parole.