MOSCOW (AFP) - Russian opposition leader Alexei Navalny was sentenced to 15 days behind bars and fined on Monday (March 27) after staging the biggest anti-corruption protests in years, an act branded a "provocation" by the Kremlin.

The United States and the European Union have voiced deep concern after Navalny and more than 1,000 others were detained in the Moscow protest on Sunday, with the State Department describing the arrests as as an "affront to democracy".

A Moscow district court ordered Navalny to serve 15 days in jail after having found him guilty of disobeying police orders. He was fined 20,000 rubles (S$488) for having organised an unsanctioned protest.

The lawyer turned activist, 40, who has announced plans to run for president next year, called Sunday's protests after publishing a report accusing Prime Minister Dmitry Medvedev of controlling a property empire through a shadowy network of non-profit organisations.

"The authorities are being accused of multi-million theft, but they remain silent," a haggard-looking Navalny said in court, insisting the protests were legal.

"More than 1,000 people were arrested yesterday but it is impossible to arrest millions," the 40-year-old lawyer said.

About 7,000 to 8,000 people demonstrated in Moscow on Sunday, according to police figures, making it one of the biggest unauthorised rallies in President Vladimir Putin's 17 years in power.

The Kremlin called the protest "a provocation and a lie", and claimed minors had been promised "financial rewards" to participate.

Demonstrations were held not just in Moscow and Russia's second city Saint Petersburg but also in a number of provincial cities where protests are rarely seen.

They attracted a significant number of minors born since Putin came to power.

"I am very happy that a generation that wants to be citizens, that isn't afraid, was born in the country," Navalny said.

Kremlin spokesman Dmitry Peskov said Russians' "civic stance" would be respected if expressed legally but, without mentioning Navalny by name, suggested "some people will continue using (politically) active people... to their own ends, calling them to illegal and unauthorised actions".

EU CALLS FOR PROTESTERS' RELEASE

Navalny was arrested as he was walking to the Moscow protest and another 1,030 people were detained, according OVD-Info, a website that monitors detentions of activists.

The vast majority were fined and released overnight, while about 120 remained in custody on Monday, OVD-Info said.

One policeman was hospitalised after suffering a head injury, the interior ministry said.

The European Union urged Russia to release the demonstrators "without delay" and expressed concern that police action had "prevented the exercise of basic freedoms of expression, association and peaceful assembly".

"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."

US State Department spokesman Mark Toner said the detention of "peaceful protesters, human rights observers, and journalists is an affront to core democratic values".

'HOPE FOR A NORMAL FUTURE ' .

The protests drew comparisons with mass anti-government rallies that swept Russia in 2011 over vote-rigging after a parliamentary election, which snowballed into the biggest challenge against Putin since he took power in 2000.

Navalny said Sunday that he was "proud" of the demonstrators.

"You are the country's best people and Russia's hope for a normal future," he wrote on Twitter.

Despite the scale of the protests, Russian state television news did not cover them, broadcasting soap operas and nature films instead.

Pro-Kremlin television host Vladimir Solovyov accused Navalny on-air Sunday of being a "paid provocateur" seeking to "destroy" the country.

The Russian constitution allows public gatherings when authorised by city authorities, but that privilege is rarely accorded to Kremlin critics.

Navalny won a surprise 27 per cent of the vote in the Moscow mayoral election in 2013, and afterwards announced plans to seek the presidency.

But he has been the subject of several legal prosecutions, and in February was found guilty of embezzlement in a case he called politically motivated. He was given a five-year suspended sentence which could make him ineligible to run in next year's vote.

Navalny and his team, though ignored by official media, have taken their anti-corruption campaign online, using social media to expose the hidden fortunes of high-ranking officials.