The separation of more than 2,000 children from their parents sparked an outcry this month, even from some allies of the Republican president.
Thousands gathered in front of the White House on Saturday to protest the Trump administration's immigration policies.
Organizers estimated 30,000 people had gathered in central Washington.
Protesters also gathered in New York, Los Angeles and scores of other US cities.
Trump says illegal immigration fosters crime and he implemented a "zero tolerance" policy in May to prosecute all immigrants apprehended for entering illegally.
Religious leaders and activists urged the administration to be more welcoming of foreigners and to reunite families.
People crowding downtown Washington waved signs and chanted "shame".