They have been galvanised in opposition to a new Texas law that severely limits access to abortions in the state.
Pro-choice supporters across the country fear that constitutional rights may be rolled back.
In the coming months, the Supreme Court is set to hear a case that could overturn Roe v Wade - the 1973 decision that legalised abortion nationwide.
In Washington DC demonstrators are heading to the Supreme Court building. The start of the rally was disrupted by some two dozen counter-demonstrators.
