Hundreds of thousands of people across the world protested the day after Donald Trump was inaugurated as America's 45th president.

What started as the Women's March on Washington spread to streets and city squares from Sydney to South Carolina on Saturday, with protesters marching in solidarity as a show of empowerment.

As many as a million people took part in the main march in Washington, DC.

Women gave a host of reasons for marching, ranging from inspiring other women to run for office, to protesting against Trump's plans to repeal the 2010 Affordable Care Act, which among other things requires health insurers to cover birth control.

Many hoped to send a unity message to Trump after a campaign in which he said Mexican immigrants were "rapists", discussed banning Muslims from entering the United States, and was revealed to have once bragged about grabbing women by the genitals and kissing them without permission.