Mahnaz Weldy marched through Tehran in support of women's rights in 1979. Now 60, the Iranian-American painter from Vienna, Va., will be doing it again, this time at the Women's March on Washington — because "a woman's battle never ends, no matter what part of the world you live in," she says.

Tamela Fish, a San Francisco-based communications director, will also be there. She'll be standing up for climate justice, she says, and to honour her late grandmother, "a strong, independent woman who would have been there for this."

Michelle Passi, a social studies teacher from Long Island, N.Y., will be in D.C., too. She'll march for her newborn son, for human rights, for women's reproductive rights and for the civic-minded students she hopes to inspire.

"It's going to be an event for the history books," Passi says.

Mahnaz Weldy

Mahnaz Weldy, 60, marched in Tehran during the Islamic Revolution. She's marching again in Washington. 'I see it as my right, as my responsibility and duty, for all my sisters in the next generation to voice my opinion. They have to fight,' she says. (Courtesy Mahnaz Weldy)

Saturday's gathering of an estimated 200,000 people is set to be the largest-ever demonstration related to a presidential election to take over the nation's capital — an impressive expectation for an event spawned by a dismayed Facebook message written on election night. (Donald Trump's inauguration on Friday, meanwhile, had been expected to draw as many as 900,000 people.)

And the five-word post that ignited all of this? "I think we should march."

The plea was tapped out by 60-year-old Teresa Shook, a retired lawyer and grandmother living in Hawaii who wrote it after the election of Donald Trump.

USA-TRUMP/WOMEN

Jayna Zweiman and Krista Suh take part in the 'pussyhat' social media campaign they created to provide pink hats for those attending the Women's March. (Lucy Nicholson/Reuters)

More than 10,000 responses greeted her the next morning. Shook's pithy online suggestion is now a full-blown national movement organized by four co-chairs.

As Shook boarded her flight to Washington on Thursday, more than 600 "sister marches" were being planned across the U.S., as well as in cities around the world, from Lethbridge, Alta., to Yangon, Myanmar.

"I'm marching for my granddaughters and women and all people who want a fair, just and inclusive world," Shook told CBC News via text message, writing from Maui.

Watch Tamela Fish describe why she's marching in Washington:

At the main event in Washington, pop singer Katy Perry, comedians Amy Schumer and Chelsea Handler, and actors America Ferrera, Scarlett Johansson, Olivia Wilde, Julianne Moore and Patricia Arquette are expected to take part.

Guest performers at the event will include Janelle Monae and Maxwell.

While related events will get underway starting at 8 a.m., the march itself kicks off on Constitution Avenue at 1 p.m.

Though it began as a specific response to Trump's win, the Women's March on Washington has since broadened its scope to preventing a dramatic clawback in women's reproductive rights and social justice issues relating to race, religion, health care, immigration and LGBT rights.

Teresa Shook

Teresa Shook, a retired lawyer and grandmother living in Hawaii, wrote a Facebook post on election night expressing her outrage over Donald Trump's victory. 'I think we should march.' By the time she woke up the next day, 10,000 people expressed interest in attending a march. (Courtesy Teresa Shook)

The message touted by many supporters of the march — and by feminists' quoting the phrase from Hillary Clinton's 1995 speech to the United Nations — is that "women's rights are human rights."

What the event is not, organizers assure, is an anti-Trump protest.

While her political views don't align with those of the new president, Nisa Cochran's core reason for being in Washington is to defend her eight-month-old daughter's future against policies she fears could be regressive and anti-woman.

"The Republicans are the ones who are really making the laws of the land. But to me it doesn't matter your political party — Republican, Democrat, Independent, whoever you are — you should want your children to have the best future, and that's why I'm marching."

Nisa Cochran

Nisa Cochran, centre, pictured with her daughter and husband, will be attending the Women's March on Washington. The 25-year-old writer says she's marching for her daughter, Eleanor. 'One day, maybe she'll sees equality. I think that's my biggest thing, it's to see equality,' she says. (Christina Tickell)

The 25-year-old writer from Virginia Beach, Va., worries about her daughter Eleanor's future access to contraception, as well as reproductive health services through Planned Parenthood, should conservative lawmakers cut funding to the organization.

The Women's March released a "guiding vision" paper last week calling for open access to "safe, legal, affordable abortion and birth control" for all — something Cochran thinks about when she considers her daughter's reproductive rights.

"What happens when she's 16 and the new administration puts in place things so that my daughter can't have freedom of choice if something horrible happens to her?"

Amanda Eriksen

Baltimore resident Amanda Eriksen, pictured with her husband, Ralph, and daughter, Sawyer, will also be participating in the Women's March alongside her aunt and cousins. (Courtesy Amanda Eriksen)

The abortion issue has caused some disunity among women interested in the event. March organizers faced a backlash after an anti-abortion group, New Wave Feminists, was named as an official partner.

Activists who RSVP'd to the march were outraged that an event created to bring attention to women's rights would affiliate itself with an anti-abortion organization. Organizers later dropped New Wave Feminists as a partner, listing their inclusion as an "error" and noting that the Women's March on Washington's stance has been pro-choice "from day one."

Some self-described feminists who oppose abortion are reportedly skipping the march for that reason, or at least they're reconsidering their attendance.

Michelle Passi

Miki Wallace, 42, pictured with her daughter, will be participating in a 'sister march' for women in Oahu, Hawaii. (Courtesy Michelle Passi)

But the opportunity to march for marginalized voices is too great to pass up for Amanda Eriksen, a 33-year-old clinical research marketer from Baltimore.

"I'm going in support of reproductive rights, LGBT rights, and my best friend's son has [cerebral palsy], so I'm thinking about disability rights," she says.

Miki Wallace, a 42-year-old financial adviser attending a sister march in Oahu, Hawaii, says that as a mother with mixed Mexican and Japanese heritage, she's troubled by some of the language used by the new president that could contribute to the persecution of minorities or the objectification of women.

Wallace says she's never been political, but after years of seeing a more progressive America take shape, she says "we're feeling like it's too many steps backward."

"Silence is consent," she says. "And I'm marching because I can't sit back and be silent."