Moscow police on Sunday detained more than 1,000 people at a protest for free municipal elections that followed a crackdown on opposition leaders.

Several opposition politicians, including Alexey Navalny, Ilya Yashin and Dmitry Gudkov, were detained ahead of the protest for organizing a rally without official authorization. Police detained 1,074 people at the rally on Saturday, RIA Novosti reported, stated the Interior Ministry.

An authorized rally a week earlier drew over 20,000 people to protest the exclusion of opposition candidates from the ballot for city council elections in September.

Investigations have been opened against 15 organizers of the protest after they ignored warnings that it lacked official approval, the Moscow city prosecutor’s office said on its website on Friday. Police issued a warning to the public not to attend the event, saying they’ll take “all necessary measures” to maintain order.