Brussels: The European Union today urged Russia "to release without delay" what it said in a statement were peaceful demonstrators detained a day earlier during nationwide protests against corruption.
Top Kremlin critic Alexei Navalny, who has announced plans to run for president in a 2018 election, was among hundreds arrested in Moscow during one of the country's biggest unauthorised rallies in recent years.
An EU spokesman said the police action had "prevented the exercise of basic freedoms of expression" association and peaceful assembly -- which are fundamental rights enshrined in the Russian constitution".
"We call on the Russian authorities to abide fully by the international commitments it has made, including in the Council of Europe... to uphold these rights and to release without delay the peaceful demonstrators that have been detained," it added.
Navalny had called for the marches after publishing a detailed report this month accusing Prime Minister Dmitry Medvedev of controlling a property empire through a shadowy network of non-profit organisations.
The report has been viewed over 11 million times on YouTube, but so far Medvedev has made no comment on the claims.
Police said the turnout at the Moscow protest was around 7,000-8,000.
Navalny was arrested as he was walking to the protest, with officers putting him in a minibus which the crowd briefly tried to block from driving off, shouting "Shame!" and "Let him out!"
Police said about 500 people had been arrested in Moscow, while OVD-Info, a website that monitors the detention of activists, said at least 933 had been detained, as well as dozens in other cities.
The Interfax news agency said 130 people were arrested in Saint Petersburg, where about 4,000 people gathered in the city centre.