Officials are saying about 500,000 people have turned out for the march protesting President Donald Trump's inauguration in Washington, double the estimated number.
The crowds could top those that turned out for Trump's inauguration on Friday.
Wearing pink, pointy-eared "pussyhats" to mock the new president, hundreds of thousands of women have marched in cities around the globe to send Donald Trump an emphatic message that they won't let his agenda go unchallenged.
"We march today for the moral core of this nation, against which our new president is waging a war," actress America Ferrera told the Washington crowd.
"Our dignity, our character, our rights have all been under attack and a platform of hate and division assumed power yesterday. But the president is not America. ... We are America and we are here to stay."
The women brandished signs with messages such as "Women won't back down" and "Less fear more love" and decried Trump's stand on such issues as abortion, health care, gay rights, diversity and climate change.
Actress Scarlet Johansson has called on women to fight for their rights in a passionate speech to the crowd.
“Let this weight not drag you down, but help to get your heels stuck in,” she declared.
The organisers' website says more than 670 marches were planned, with more than 2 million marchers expected to protest against Trump, who was sworn in as the 45th US president on Friday (local time).
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Organisers, said an estimated 100,000 people descended on central London on Saturday while 200,000 people gathered for a rally outside the US Capitol building.
Organisers called for people to join them "as part of an international day of action in solidarity" on Trump's first full day in the Oval Office.
Celebrities Katy Perry, Scarlett Johansson, Amy Schumer and Patricia Arquette are expected to be among the demonstrators at the Washington event.
Ugly Betty star America Ferrera, who helped organise the march, told the crowd: "It's been a heartbreaking time to be both a woman and immigrant in this country.
"The platform for hate and division assumed power yesterday.
"But the president is not America. His cabinet is not America. Congress is not America. We are America and we are here to stay."
Defeated presidential candidate Hillary Clinton has tweeted her support, making her feelings clear.
Many of the women in Washington wore pink knitted hats with cat ears - a reference to comments made by Trump in a 2005 leaked video in which he bragged about grabbing women "by the pussy".
Washington subway trains and platforms were packed with people.
The Metro sent a service alert warning of "system-wide delays due to extremely large crowds."
At least one station was closed to new passengers because of the crowds backed up on the platform.
Critics of the marches say they are a divisive style of "identity politics," but Democratic Congressman Jim Himes of Connecticut, a supporter of the marchers, rejected that assertion.
"It is Donald Trump who singled out Muslims for a Muslim registry. It was Donald Trump who made disparaging comments about women. It was Donald Trump who criticized a judge of Mexican heritage," he said.
"That's identity politics. We're sending the message that we're all Americans."
Marches also took place in other UK cities including Manchester, Edinburgh, Belfast, Liverpool and Cardiff, with thousands turning out.
Beginning at the American Embassy in London, the London Women's March made its way around the streets of the capital and to a rally in Trafalgar Square.
In Europe, marches also took place in Berlin, Paris, Rome, Vienna, Geneva and Amsterdam.
Around 2,000 people marched in Vienna, according to estimates by the police and organisers, but sub-zero temperatures quickly thinned the crowd to a couple of hundred.
In Africa, hundreds of protesters in Nairobi's Karura Forest waved placards and sang American protest songs.
In Sydney, about 3,000 people - men and women - gathered for a rally in Hyde Park before marching on the US consulate, while organisers said 5,000 people rallied in Melbourne.
Protests have even reached as far as Antarctica, where a group of international people on an expedition ship in Antarctica are marching on their ship "in a sign of solidarity with the Women’s March on Washington, and other sister marches around the world", according to their post on actionnetwork.org.
Reuters / Newshub.