MOSCOW (AP) — At least seven opposition activists who have been barred from running in the Moscow legislature election have been summoned for questioning, as authorities ramp up pressure on the opposition ahead of a major rally on Sunday.

Police searched the homes of two activists late Wednesday and summoned seven for questioning. Earlier Wednesday, a Moscow court sent opposition leader Alexei Navalny to jail for 30 days for calling an unsanctioned protest.

The Russian Investigative Committee said it has launched an inquiry into efforts to hamper electoral process following numerous protest rallies. The activists are currently treated as witnesses.

Protests erupted in Moscow earlier this month after election authorities refused to register a number of opposition candidates for the Moscow legislature elections in September, citing minor violations.