Russian police forcibly detained 1001 people attending a protest in Moscow on Saturday to demand free elections, including prominent activist Lyubov Sobol, after authorities warned the demonstration was illegal.

Police removed Sobol, an ally of jailed opposition politician Alexei Navalny, from a taxi and bundled her into a van minutes before the start of what anti-Kremlin activists described as a peaceful walk to protest against the exclusion of their candidates from an election next month.

In another blow to Navalny, Russian investigators opened a criminal investigation into the alleged laundering of 1 billion rubles ($15.3 million) by his anti-corruption foundation. Navalny and his allies say the foundation, which has published a slew of embarrassing investigations into government officials, is transparently financed from public donations.

OVD-Info, an independent monitoring group, said police had detained 1,001 people in Moscow on Saturday, in some cases beating them with truncheons as they lay on the floor. Reuters reporters witnessed dozens of arrests. In one case police carried off a man as he clung upside down to his bicycle.

Police said they had detained 600 and that 1,500 had attended the protest, though footage of demonstrations which flared in different parts of Moscow suggested many more had taken part. Opposition activists later posted screen shots online of police reports suggesting around 10,000 had attended.