MOSCOW - Tens of thousands of Muscovites staged Russia's biggest political protest for years on Saturday, rallying to demand free city-wide elections in spite of a government crackdown.

Hours before the demonstration, police detained one of the leading opposition activists, Lyubov Sobol, who is on a hunger strike. Masked men raided her office and the police said they had information she and other activists were plotting a "provocation" at Saturday's rally.

Protesters filed through metal detectors with flags and banners, staging their fourth major protest in a month calling for opposition-minded candidates forced off the ballot to be allowed to run in a city election next month.

Moscow officials authorized Saturday's rally, unlike last weekend when police detained more than 1,000 people, sometimes violently, at an unauthorized demonstration.

The White Counter monitoring group said it had counted 40,000 people at the rally. Police estimated turnout at 20,000. Russia has not seen sustained opposition demonstrations on this scale since 2011-2013 when protesters took to the streets against perceived electoral fraud.

Investigators have opened criminal proceedings against about a dozen people for what they say was mass civil unrest at earlier protests, a crime that carries a heavy jail term. They have also opened a money-laundering investigation into Kremlin critic Alexei Navalny's anti-corruption foundation.

Ahead of Saturday's protest, masked police searched an office used by opposition activist Sobol and took her in for questioning, she wrote on Twitter.

"I won't make it to the protest. But you know what to do without me....Russia will be free!" Sobol said.

Throngs of protesters, sheltering from rain under umbrellas, chanted "freedom for political prisoners" and "Russia will be free".

Some pro-Kremlin politicians and officials have suggested that the West has helped orchestrate the protests. Navalny and at least seven of his allies are currently in jail for breaking protest law.

Several of Russia's most popular musicians have said they want to perform at Saturday's protest, which the authorities have rejected.

Some opposition activists have also said they plan to walk through central Moscow afterwards, setting up a possible confrontation with police who say they will not tolerate trouble.

Navalny allies are also staging protests in other Russian cities. A Reuters reporter saw about 10 people being detained at a rally in the city of St Petersburg.

The focus of protesters' anger is a prohibition on a slew of opposition-minded candidates, some of whom are allies of Navalny, from taking part in a September election for Moscow's city legislature.

That vote, though local, is seen as a dry run for a national parliamentary election in 2021.

Authorities say the opposition candidates failed to collect enough genuine signatures to register. The excluded candidates say that is a lie and insist on taking part in a contest they believe they could win.

The ruling United Russia party's popularity rating is at its lowest since 2011 and President Vladimir Putin's own rating has fallen due to discontent over falling living standards.

At well over 60 percent, it is still high compared to many other world leaders however, and last year the 66-year-old former KGB intelligence officer won a landslide re-election and a new six-year term until 2024. -Reuters