WASHINGTON – Hundreds of thousands are organizing their forces to attend the Women’s March on Washington one day after Donald Trump’s inauguration, in what is anticipated to become the largest demonstration in the American history.

“A march of this magnitude, across this diversity of issues has never happened before,” Kaylin Whittingham, president of the association of black women attorneys, told The Guardian.

“We all have to stand together as a force no one can ignore.”

The call to the women march started as a spontaneous feminist rallying cry via social media.

The march is planned on January 21, just one day after president-elect Trump’s inauguration.

It will be attended by almost 200 progressive groups, which represent varied fields, including environment, legal abortion, prisoners’ rights, voting rights, a free press, affordable healthcare, gun safety, racial and gender equality and a higher minimum wage.

More than 300 simultaneous local protests will also occur, across all 50 states, and support marches are planned in 30 other countries, organizer Linda Sarsour said.

“We have no choice. We need to stand up against an administration that threatens everything we believe in, in what we hope will become one of the largest grassroots, progressive movements ever seen,” Sarsour, an American Muslim activist, said.

The plan for the march started on November 9 when Teresa Shook, a retired lawyer in Hawaii, posted on Facebook suggesting a protest timed around Trump’s inauguration.

The protest was a direct reaction to Trump’s shock win and his comments and actions related to women.

The message ended up on Pantsuit Nation, one of the invitation-only Facebook support groups lauded by Hillary Clinton in her concession speech.

As support surged, it was called the Million Women March, which sparked some anger because of its echoes of the Million Man March, in Washington in 1995, and the Million Woman March, in Philadelphia in 1997, both organized as predominantly African American demonstrations to protest against racism.

“The presidential election was on the Tuesday and I came in on the Friday,” said Sarsour, who is also a civil rights activist in New York and an Arab American with Palestinian roots.

Growing Support

Changes were quickly made to the event after gun control campaigner Tamika Mallory, who is black, and Carmen Perez, a Latina and civil rights worker, joined the leadership team, alongside female New York fashion designer Bob Bland.

“Some people think we are tokens, but I’m not just a pretty Muslim face – we’re leading this together,” said Sarsour, who is in charge of fundraising.

Celebrities slated to attend include Scarlett Johansson, America Ferrera, Uzo Aduba, Zendaya, Katy Perry and Cher.

“It’s never happened that so many people have gathered in opposition to the new administration on day one,” said Sarsour.

“Will it be the largest US mass mobilization ever? I’ll be able to tell you on January 22.”

Thanu Yakupitiyage, spokeswoman for the advocacy group and march partner, the New York Immigration Coalition, is marching not just for immigrants’ rights but for women’s equality, fair police reforms and healthcare protection, she said.

“A lot of immigrant communities were scared by Trump’s vitriolic messages. Some who are undocumented or insecure may be afraid to march in Washington,” she said.

Colleen Flanagan will have to navigate her wheelchair amid seething masses of marching women.

“I may not be marching but I will personally be rolling in Washington – for all women,” said Flanagan, a Boston-based consultant on policy for the disabled.

Following its rapid expansion in scale and scope, the march organizers on Thursday published the event’s new set of “unity principles”.

“It adds up to a comprehensive call for social justice and equal rights,” said Jessica Neuwirth, a human rights lawyer and president of a leading partner group, the Equal Rights Amendment Coalition.

Jon O’Brien, who will attend the march as president of event partner Catholics For Choice, said the march is about “true solidarity”.

“There will be all kinds of people there,” he said.

“White, black, LGBT, straight, Democrat, moderate Republican, rich, poor – in other words, America.”