Thousands of rallies are expected across the US on Saturday in the latest "No Kings" protest against the policies of President Donald Trump and his administration.

Organizers say that more than 3,200 events are planned in all 50 states for what they hope could be the largest single-day nonviolent protest in US history. The two previous No Kings events attracted millions of participants.

Flagship rallies will take place in New York, Los Angeles, Washington, D.C., and Minnesota's Twin Cities, but two-thirds of participants are expected from outside major city centres, a nearly 40 per cent jump for smaller communities from the movement's first mobilization last June, organizers said.

"The defining story of this Saturday's mobilization 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.

With midterm elections later this year in the US, organizers say they've seen a surge in the number of people organizing events and registering to participate in deeply Republican states such as Idaho, Wyoming, Montana and Utah. Competitive suburban areas that have helped decide national elections are seeing "huge" increases in interest, Greenberg said, citing Pennsylvania's Bucks and Delaware counties, East Cobb and Forsyth in Georgia, and Scottsdale and Chandler in Arizona as examples.

"Voters who decide elections, the people who do the door knocking and the voter registration and all of the work of turning protests into power, they are taking to the streets right now, and they are furious," she said.

In a statement, White House spokeswoman Abigail Jackson dismissed the rallies as "Trump Derangement Therapy Sessions" of interest only to journalists. Saturday will mark the third No Kings Day of Action. The movement launched last year on Trump's birthday, June 14, and drew an estimated 4 to 6 million people spread across roughly 2,100 sites nationwide. The second mobilization in October involved an estimated 7 million participants across more than 2,700 cities, according to a crowd-sourcing analysis published by prominent data journalist G. Elliott Morris.

That October event was largely fueled 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 protest comes amid what organizers called a call to action against the bombardment of Iran by the U.S. and Israel, a conflict that is now four weeks old.

Deirdre Schifeling, chief political and advocacy officer for the American Civil Liberties Union, said protests have led to tangible results. "Whenever we stand up to President Trump's abuses of power, like most bullies, he backs down," she said, citing administration reversals following earlier demonstrations over National Guard deployments in Los Angeles and ICE killings of two American citizens in Minneapolis.