More than 3200 events were planned in all 50 states on Saturday. The two previous No Kings events attracted millions of participants.
In Minnesota, a flashpoint in Trump's crackdown on illegal immigration, a massive rally was held outside the state capitol building in Saint Paul.
Many in the crowd held aloft posters bearing photos of Renee Good and Alex Pretti, whom federal immigration officers fatally shot in Minneapolis this year.
Minnesota Governor Tim Walz, the Democratic vice presidential nominee in 2024, told the crowd their resistance to Trump and his policies makes them "the heart and soul" of everything good about the US.
"They call us radicals," Walz said on Saturday.
"You're damn right we've been radicalised - radicalised by compassion, radicalised by decency, radicalised by due process, radicalised by democracy, and radicalised to do all we can to oppose authoritarianism."
US Senator Bernie Sanders of Vermont, a Trump critic who sought the Democratic presidential nomination in 2016 and 2020, was another speaker at the event in Minnesota.
Bruce Springsteen also appeared and performed his song Streets of Minneapolis - a ballad that blasts Trump's immigration crackdown and laments the deaths of Good and Pretti.
"We will not allow this country to descend into authoritarianism or oligarchy in America," said Sanders.
"We, the people, will rule."
Other large rallies took place in New York, Dallas, Los Angeles, Philadelphia and Washington, but two-thirds of the events were happening outside major cities, a nearly 40 per cent jump for smaller communities from the movement's first mobilisation last June, organisers said.
In New York, a crowd that police estimated at tens of thousands stretched more than 10 blocks in midtown Manhattan.
Robert De Niro, one of the organisers, said no president before Trump had posed "such an existential threat to our freedoms and security".
On the National Mall in Washington, the crowd chanted pro-democracy slogans and held anti-Trump signs.
Outside one high-rise assisted-living centre in Chevy Chase, Maryland, a group of elderly people in wheelchairs held signs encouraging passing cars to "Resist tyranny", "Honk if you want democracy" and "Dump Trump".
Thousands attended a Dallas event that had clashes between No Kings demonstrators and counter protest groups, including one led by Enrique Tarrio, the former leader of the far-right organisation the Proud Boys.
Minor scuffles erupted when counter protesters blocked streets. Dallas police made several arrests.
Trump's approval rating has fallen to 36 per cent, its lowest point since his return to the White House, according to a Reuters/Ipsos poll.
A spokesperson for the National Republican Congressional Committee criticised Democratic politicians and candidates for supporting the rallies.
"These Hate America Rallies are where the far-left's most violent, deranged fantasies get a microphone and House Democrats get their marching orders," spokesperson Mike Marinella said in a statement.
With midterm elections later this year in the US, organisers say they have seen a surge in the number of people organising anti-Trump events and registering to participate in deeply Republican states like Idaho, Wyoming, Montana and Utah.
The "No Kings" movement launched last year on Trump's birthday, June 14, drawing an estimated four-to-six million people across roughly 2100 sites nationwide.
The second mobilisation in October involved an estimated seven million participants in more than 2700 cities.
That October event was largely fuelled by a backlash against a government shutdown, an aggressive crackdown by federal immigration authorities, and the deployment of National Guard troops to major cities.
Saturday's events come amid what organisers said was a call to action against the bombardment of Iran by the US and Israel, a conflict that is now four weeks old.
Morgan Taylor, 45, attended the Washington protest with her 12-year-old son, and said she was enraged by Trump's military action in Iran, which she called a "stupid war".
"Nobody's attacking us," Taylor said.
"We don't need to be there."
