There were more than 2600 rallies in major cities on Saturday in small towns as well as some in foreign capitals, challenging a Trump-led agenda that has reshaped the government and upended democratic norms with unprecedented speed since he took office in January.
"There is nothing more American than saying, 'We don't have kings' and exercising our right to peacefully protest," said Leah Greenberg, co-founder of Indivisible, a progressive organisation that planned the protests.
Earlier on Saturday, hundreds of Americans gathered in major European cities like London and Paris.
The protests reflect growing unease among many Americans, mainly on the left of the political spectrum, to developments such as the criminal prosecution of Trump's perceived political enemies, his militarised immigration crackdown and the sending of National Guard troops into US cities - a move Trump has said was aimed at fighting crime and protecting immigration agents.
As his administration has tried to rapidly implement its policies, Trump has installed inexperienced loyalists across the ranks of his administration and sought to apply pressure on the news media, law firms and higher education.
The rallies were boisterous but orderly, with police largely keeping a low profile.
In Washington, demonstrators filled the street as they marched toward the US Capitol, chanting and carrying signs, US flags and balloons. Many people - and their dogs - wore costumes in a peaceful, carnival-style atmosphere.
Four marchers dressed in prison stripes and large caricature heads of Trump and other officials displayed a sign saying "Impeach Trump Again".
Protester Aliston Elliot, wearing a Statue of Liberty headpiece and holding a "No Wannabe Dictators" sign, said: "We want to show our support for democracy and for fighting (for) what is right. I'm against the overreach of power."
Events in New York City, Boston, Chicago and Atlanta also drew large crowds. In downtown Houston, US Marine Corps veteran Daniel Aboyte Gamez, 30, joined a crowd that officials said numbered at least 1500.
"I don't understand what's going on in this nation right now," said Gamez, who served in Iraq, Afghanistan and Syria.
"As a Marine Corps vet, I understand that the United States was founded upon action against tyrants, against kings."
Kevin Brice, 70, a military veteran among thousands of protesters streaming into the riverfront area of Portland, Oregon, wore a black sweatshirt emblazoned with the slogan "No Kings since 1776".
"I'm embarrassed that we have federal agents in masks arresting people in the streets. I'm embarrassed that we're talking about using the military against civilians. I'm embarrassed that it's OK to lie and make stuff up," Brice said.
"So even though I'm a lifelong Republican, I don't support the direction the party is going."
Trump has called the protests "hate America" rallies and in an interview with Fox Business said that "they're referring to me as a king - I'm not a king".
In June, 2000-plus "No Kings" protests took place, mostly peacefully, on the same day that Trump celebrated his 79th birthday and held a military parade in Washington.
Dana Fisher, a professor at American University in Washington and author of several books on American activism, forecast that Saturday could see the largest protest turnout in modern US history.
She expected that more than three million people would take part based on registrations and participation in the June events.
