Tens of thousands of Russians staged what a monitoring group called the country’s biggest political protest for eight years on Saturday, defying a crackdown to demand free elections to Moscow’s city legislature.

Police rounded up scores of people after the demonstration in Moscow and at another rally in St Petersburg, and detained a leading opposition figure before it began.

The White Counter monitoring group said up to 60,000 people had attended the Moscow rally, describing it as the biggest in Russia for eight years. Police put turnout at 20,000.

Russia’s state communications watchdog has asked Google to stop advertising “illegal mass events” on its YouTube video platform, it said on Sunday.

Multiple YouTube channels broadcast the event live.

The watchdog, Roscomnadzor, said some entities had been buying advertising tools from YouTube, such as push notifications, in order to spread information about illegal mass protests, including those aimed at disrupting elections.