More than 18,000 Russians have rallied in Moscow to demand the release of protesters jailed in what Kremlin opponents says is a campaign to stifle dissent.

Key points: Several protesters have been jailed for up to four years

Several protesters have been jailed for up to four years In response to anger over the sentences, authorities have released one jailed protester and dropped charges against another

In response to anger over the sentences, authorities have released one jailed protester and dropped charges against another The protesters were arrested during demonstrations over opposition candidates not being allowed to run in a recent election

Several demonstrators were arrested during protests, started in July, against the exclusion of many opposition candidates from a local election and allegations of police brutality.

Several people have been sentenced to up to four years' jail, and others are being prosecuted for crimes such as violence against police officers.

Seen as harsh jail sentences by many Muscovites, they have sparked an unusual public outcry.

In a rare step following the outcry, courts freed one person on bail and dropped charges against another.

But President Vladimir Putin's opponents say the moves may have been a tactic to avoid wider concessions, and they want to step up pressure for others to be freed.

Russian opposition activist Lyubov Sobol speaks during the rally in Moscow. ( AP: Dmitri Lovetsky )

Prior to the demonstration on Sunday Opposition politician Leonid Volkov said a strong turnout would compel the Government to release more people.

"If there are 50,000 people, they'll let everyone out," Mr Volkov posted on Twitter.

Protesters filed through metal detectors under light rain, waving flags from an array of opposition groupings of starkly different political stripes.

A crowd of 18,000 people had been counted 30 minutes after the start of the rally, according to the White Counter group which monitors political protests.

The rally has been authorised by the Moscow mayor's office, meaning mass detentions by police are less likely to happen.

People hold signs saying "Freedom to Yevgeny Kovalenko" and "Freedom to Ivan Podkopaev". ( AP: Dmitri Lovetsky )

The series of protests began in July when more than a dozen opposition-minded candidates were not allowed to run in a September 8 election to Moscow's city legislature on a technicality.

Police say people were detained or prosecuted for breaking the law and the protests had to be dispersed as they had not been authorised and were illegal.

After his allies were barred from the vote, Opposition Leader Alexei Navalny called on supporters to vote tactically for opponents of the ruling United Russia Party, regardless of their political stripe.

United Russia, which supports Mr Putin, lost a third of its seats in the Moscow city assembly, a setback for the authorities that Mr Navalny said was a victory for the Kremlin's opponents even though the governing party kept its majority.

The rallies were the largest sustained protest movement in Moscow in almost a decade, peaking at around 60,000 people before appearing to lose momentum.

Reuters