More than 3200 events are planned in all 50 US states and several cities outside the United States.
The two previous No Kings events attracted millions of participants.
Singers Bruce Springsteen and Joan Baez will headline a rally at the state capitol in Minnesota, where upward of 100,000 people are expected to gather in an area that became a flashpoint over Trump's crackdown on illegal immigration and the incursion of federal immigration agents into Democratic-led urban centres.
Other large rallies are taking place in New York, Los Angeles and Washington DC but two-thirds of the events are happening outside major city centres, a nearly 40 per cent jump for smaller communities from the movement's first mobilisation last June, organisers said.
On the National Mall in Washington DC, 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".
In Austin, Texas, a brass band provided the soundtrack as protesters gathered outside City Hall before a march through downtown.
Thousands gathered in midtown Manhattan where actor Robert De Niro, one of the organisers, said that "there have been other presidents who have tested the constitutional limits of their power but none have been such an existential threat to our freedoms and security".
"The defining story of this Saturday's mobilisation is not just how many people are protesting but where they are protesting," said Leah Greenberg, co-founder of Indivisible, the group that started the No Kings movement last year and led planning of Saturday's events.
A spokesman 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," spokesman 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.
In northern Virginia just outside Washington DC, several hundred people began gathering on Saturday close to Arlington National Cemetery before a planned march across the Potomac River to the capital city's National Mall.
Some passing drivers honked their horns in support but others slowed down to berate the protesters.
"You're all idiots," one man shouted from his car.
John Ale, 57, a retired air-conditioning and heating contractor, said he drove 20 minutes from his home in Virginia to join the march.
"What's happening in this country is unsustainable," he said.
"The middle class, the little people, can't afford to live anymore. And he (Trump) is breaking the norms, the things that made us function as a country."
