Hundreds of thousands were expected to participate in the evening marches near Seoul's presidential palace and the Constitutional Court. Park's supporters are planning their own rallies in nearby streets.
The court has up to six months to decide whether Park should permanently step down over a corruption scandal or be reinstated. The judges said yesterday that Park cannot be forced to testify in the impeachment trial as it enters its argument phase next week.
State prosecutors have accused Park of colluding with a longtime confidante to extort money and favours from the country's largest companies and allowing the friend to manipulate her administration. Park has apologised for putting faith in her jailed friend, Choi Soon-sil, but has denied any legal wrongdoing.
State prosecutors have now handed over the investigation to a special prosecution team, which has been focusing on proving bribery suspicions between Park and the Samsung Group. The business giant is suspected of sponsoring Choi in exchange for government favours.
The deal shaved the fund's stake in one of the companies by an estimated hundreds of millions of dollars in value, but allowed Samsung scion Lee Jae-yong to promote a father-to-son succession of leadership and increase corporate wealth at the group.
Investigators are also looking into allegations that Park's administration blacklisted thousands of artists for their political beliefs.