Organizers say more than 2,500 events were planned across the country to peacefully protest the Trump Administration's policies.
The march in Philadelphia started on the north side of City Hall in the early afternoon.
The march ended with a rally at Independence Mall, which will feature more than a dozen speakers.
Organizations across the country gathered to create the first No Kings Day rally in June.
The group Indivisible Philadelphia says 100,000 people attended that event.
Dozens of protests happened in surrounding counties, too.