MOSCOW (AP) — Police have established a heavy presence at the Moscow mayor's office ahead of an expected protest rally and several opposition figures have been detained.

The unauthorized demonstration planned for later Saturday in Moscow aims to protest authorities' refusal to put several opposition members on the ballot for September's city council election. The decision to bar the candidates for allegedly insufficient signatures on nominating petitions sparked several days of demonstrations this month.

Alexei Navalny, Russia's most prominent opposition activist, had called Saturday's protest, saying it would continue until the rejected candidates are allowed to run. Navalny was jailed for 30 days Wednesday for calling an unauthorized protest.

A top Navalny associate, Ivan Zhdanov, said Saturday on Twitter that he had been detained. Ilya Yashin, another leading opposition member, also has been detained.