Hundreds of thousands took to the streets of Washington one day after President Trump's inauguration to protest the new president and advocate for women's causes.

The turnout for the Women’s March on Washington was so massive that it filled up the entire march route at once, according to The Associated Press.

As a result, organizers won’t lead a formal march along the route, a D.C. official told the AP, although the crowd will still head toward the White House.

The official told the AP there could be more than a half-million people at the rally in Washington. March organizers, who originally sought space for 200,000 protesters, revised their own estimation Saturday morning to north of 500,000.

Protesters with signs and pink knitted hats filled the city’s Metrorail trains and streets as they made their way to Capitol Hill. The event, which kicked off with a rally of celebrity and political guest speakers, began at 10 a.m., and the march through D.C. streets was slated begin at 1:15 p.m.

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The march, which began with a single Facebook post, has become the premier event for Trump’s opponents during the inauguration weekend and has spurred 673 “sister marches” around the U.S. and the world, according to the organizer’s website.

Some of the largest demonstrations outside D.C. are being held in New York City, Boston, and Chicago.

Sen. Elizabeth Warren Elizabeth WarrenWarren, Schumer introduce plan for next president to cancel ,000 in student debt The Hill's 12:30 Report - Presented by Facebook - Don't expect a government check anytime soon No new taxes for the ultra rich — fix bad tax policy instead MORE (D-Mass.), a harsh critic of Trump during the presidential campaign, spoke to rally attendees in Boston on Saturday, calling on them to fight back against Trump.

“The fact is that the playing field has been tilted badly in favor of those at the top for a generation now,” Warren told thousands of people at the Boston Common.

“And now, President Trump and the Republican Congress are ready to ram through laws that will tilt it even harder.”

Countries from India and Iraq to Germany and Mexico are also slated to host sister marches.

Though it never mentions Trump by name, the march’s mission statement cites the “rhetoric of the past election cycle” as it calls for unified grassroots change.

“The Women’s March on Washington will send a bold message to our new government on their first day in office, and to the world that women's rights are human rights,” the website reads.

Speakers included feminist icon Gloria Steinem, an honorary co-chair of the event.

Steinem’s message for Trump was “do not try to divide us,” which she delivered to a rousing swell of cheers from the crowd.

"If you force Muslims to register, we will all register as Muslims,” Steinem said.

Other speakers include Planned Parenthood Federation of America’s President Cecile Richards. Richards, who campaigned prominently for Hillary Clinton Hillary Diane Rodham ClintonHillicon Valley: FBI chief says Russia is trying to interfere in election to undermine Biden | Treasury Dept. sanctions Iranian government-backed hackers The Hill's Campaign Report: Arizona shifts towards Biden | Biden prepares for drive-in town hall | New Biden ad targets Latino voters FBI chief says Russia is trying to interfere in election to undermine Biden MORE, is leading the fight against GOP efforts to cut all federal funding of the organization.

A number of lawmakers also appeared, including Sens. Cory Booker (D-N.J.) and Kirsten Gillibrand Kirsten GillibrandSuburban moms are going to decide the 2020 election Jon Stewart urges Congress to help veterans exposed to burn pits The Hill's Campaign Report: 19 years since 9/11 | Dem rival to Marjorie Taylor Greene drops out | Collin Peterson faces fight of his career | Court delivers blow to ex-felon voting rights in Florida MORE (D-N.Y.), who took pictures with rallygoers, and Sen. Kamala Harris (D-Calif.), who spoke at the rally.

Celebrities like Michael Moore, Ashley Judd and America Ferrera also spoke from the stage.

Updated at 1:46 p.m.