Thousands are gathering in cities across the country in solidarity with the Women's March in Washington, D.C.. (Reuters)

Hundreds of thousands of demonstrators on Saturday packed the streets of U.S. cities from Los Angeles to Boston, donning their pink hats beneath palm trees and falling snow, and spilling so far beyond capacity in some places that authorities were forced to transform planned Women’s Marches into standing rallies.

With their chants, signs and banners for women’s and minority rights — in many cases, phrased as direct challenges to President Trump — activists said Saturday’s marches formed the largest wave of opposition to a new president in modern American history.

“The country is really fired up,” said Kate Lagreca, one of the organizers in Boston, where she said an estimated 125,000 people showed up–45,000 more than they were expecting. “We are sending a message to politicians. I’m really glad that at a grassroots level we got hundreds of thousands of people involved.”

Organizers said a quarter million people showed up in Chicago alone, forcing authorities to transform a planned march through the Windy City into a standing rally, after a downtown park reached its overflow point. (Thousands kept marching past the park anyway.) In Los Angeles, a police spokesman said authorities had to temporarily shut down additional side streets after the crowds swelled beyond the space available for the planned downtown march. And in Juneau, Alaska, an observer marveled that the crowd was the biggest he had ever seen on the state Capitol’s steps.

“Staying home was not an option,” said Suze Anderson, 41, who marched in Chicago. “We can’t let Trump think anything about his presidency is normal.”

Like the Women’s Marchers in Washington, demonstrators across the country hoisted signs in support of women’s, immigrant and LGBTQ rights, Planned Parenthood, Black Lives Matter, wage equality and environmental protection — all causes that Trump has appeared to oppose. They protested against gun violence, bigotry, discrimination and sexual assault. They lambasted Trump’s rhetoric and turned it against him.

“This Mexican pussy grabs back!” read a sign spotted on the New York City subway. “Sad!” read a one-word sign in St. Louis.

Self-identification as a “nasty woman” — words Trump used to describe Hillary Clinton — was everywhere. So were images of uteri and fallopian tubes.

The Women’s March on Washington website listed 673 planned marches around the world, including hundreds in cities and towns across the United States that ranged from major metropolises like Los Angeles and Chicago to tiny towns like Lander, Wyo., and Stanley, Idaho.

They included nearly 50 marches in California alone, and 18 planned in Alaska, where temperatures in some places hovered at 15 degrees below zero. In Las Vegas, they marched past palm trees and casinos. In Idaho, they marched through falling snow. In Jackson, Miss., it was warm enough that families and children sported T-shirts. There were four marches in the U.S. territory of Puerto Rico.

As they rallied, many of the marchers took to social media, apparently amazed by the sheer numbers that turned out in their cities and towns, particularly in states like Kentucky and Idaho that voted for Trump, and in tiny towns like Marfa, Texas and Brookings, Oregon that rarely muster protests.

“The #WomensMarch in Juneau is the biggest demonstration I’ve ever seen on Alaska’s Capitol steps,” tweeted Austin Baird in Juneau.

“WOMENS march on Idaho. #wow,” tweeted Melissa Wintrow in Boise.

In Chicago, a police spokesman said the city’s Grant Park had reached capacity by noon, so authorities “transitioned to a support rally,” said Officer Jose Estrada.

Capt. Andrew Neiman of the Los Angeles Police Department said: “We are doing our best to facilitate because they are squeezing into every street right now.”

Despite the size of the crowds, authorities said the protests were proceeding peacefully.

The organizers have ranged from a writer and photographer in Portland, Ore., to a pair of teenagers in Boise, Idaho, according to local news reports.

In Birmingham, Ala., where immigrants rights groups and NAACP leaders were addressing the crowd, march organizer Shante Wolfe-Sisson listed the demonstration’s priorities to a local television station. “We stand for unity, we stand for reproductive rights, we stand for equal access to resources for those who may be HIV positive, for those who may need access to other resources that today we are currently trying to tell our legislators are relevant for women,” Wolfe-Sisson said.

A group of women in St. Louis marched through 60 degree weather with their protest slogans scrawled across their shirtless torsos. In Las Vegas, they carried rainbow flags and “nasty women fight back” signs past palm trees and casinos.

Celebrities showed up in marches outside of Washington, too. Actor Charlize Theron and comedian Chelsea Handler led the march in Park City, Utah, where actors have gathered for the Sundance Film Festival. Actor Rowan Blanchard addressed the marchers in Los Angeles, and Seth Rogan tweeted from the crowd in New Orleans.

Federal and local lawmakers including Sens. Claire McCaskill (D-Mo.), Cory Booker (D-N.J.), Benjamin L. Cardin (D-Md.) and Reps. Barbara Lee (D-Calif.) and Al Green (D-Tex.) also showed up in pictures on social media.

The Women’s March on Washington website estimated that attendance of marches outside Washington would top 2.5 million.

For those who couldn’t make it out into the streets, march organizers promoted the online Disability March.

Herman Wong in Washington D.C. and Mark Guarino in Chicago contributed to this report.