A Seoul police official says about 14,000 people have taken part in the rally, which began at a square in front of City Hall.
The demonstrators are planning to march toward an area near a hospital where a 70-year-old protester remains in a coma after being injured during a large anti-government rally in Seoul on November 14, when dozens were hurt in clashes with police.
Park's government has clamped down on labor and civic groups resisting her drive to make labor markets more flexible and arrested several labor union officials involved in organising some protests that spiraled into violence last year.