Thousands packed New York City's iconic Times Square and streets all around, with people holding signs with slogans like "Democracy not Monarchy" and "The Constitution is not optional".
Ahead of the demonstrations, Trump allies accused the protesters of being linked with the far-left Antifa movement, and condemned what they called "the-hate America rally".
Several US states had mobilised the National Guard. But organisers said the events, which drew nearly seven million people, were peaceful.
Since returning to the White House in January, Trump has expanded the scope of presidential power, using executive orders to dismantle parts of the federal government and to deploy National Guard troops to US cities despite objections by state governors.
He has also called on the administration's top law enforcement officials to prosecute his perceived enemies.
The president says his actions are necessary to rebuild a country in crisis and has dismissed accusations that he is a behaving like a dictator or fascist as hysterical.
But critics warn some of the moves by his administration are unconstitutional and a threat to American democracy.
In an interview with Fox News, set to air on Sunday, Trump appeared to address the rallies.
"A king! This is not an act," Trump said in a preview clip of the interview. "You know – they're referring to me as a king. I'm not a king."
In New York, sections of the crowd regularly erupted into chants of "This is what democracy looks like" as a near-constant drumbeat boomed in the background.
Helicopters and drones could be seen flying overhead, and police stood on the sidelines.
The New York Police Department said more than 100,000 people had gathered across all five of the city's boroughs, and that no protest-related arrests were made.
In Times Square, a police officer standing by estimated that over 20,000 were marching down 7th Avenue.
