The protests marked the largest outpouring of opposition to Trump's presidency since he returned to power in January, and came the same day that thousands of military personnel, vehicles and aircraft will march through and fly over Washington, DC, in a parade celebrating the US Army's 250th anniversary.
Saturday is also Trump's 79th birthday.
The Republican president has ordered National Guard troops and US Marines to Los Angeles, a heavily Democratic city - a deployment that California Governor Gavin Newsom has challenged in court.
Anti-Trump groups planned nearly 2000 demonstrations of varying sizes across the country to coincide with the parade. Many are taking place under the theme "No Kings," asserting that no individual is above the law.
All planned "No Kings" protests in Minnesota were canceled following news a Democratic politician had been killed in what Governor Tim Walz described as a "politically motivated assassination".
People heading into Washington for the parade encountered a massive security presence, with some 30 km of 2.4m-high black fencing, much of it reinforced with concrete traffic barriers, cordoning off streets and surrounding landmarks including the Washington Monument.Â
The celebrations will cost the US Army between $US25 million and $US45 million ($A38 million to $A69 million), US officials have told Reuters. That includes the parade itself as well as the cost of moving equipment and housing and feeding the troops.
Critics have called the parade an authoritarian display of power that is wasteful, especially given Trump has said he wants to slash costs throughout the federal government.
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