Organisers said attendees are "hurting and scared" as President Trump takes office - and are calling for a greater female voice in political life.
Tens of thousands of women from across the US will march in Washington today - with several hundred ‘sister marches’ to be held across the world in solidarity.
Organisers said they aim to send a message to the new US President that women’s rights need defending - and challenge the “hateful rhetoric” of the incoming president.
More than 200,000 people are expected to attend the Washington gathering.
The organiser’s mission statement said attendees are "hurting and scared" as President Trump takes office - and are calling for a greater female voice in political life.
"In the spirit of democracy and honouring the champions of human rights, dignity, and justice who have come before us, we join in diversity to show our presence in numbers too great to ignore," the statement says.
Women and other groups from across the world will demonstrate in solidarity with the Washington march - with gatherings already held in Myanmar, New Zealand and Australia.
In Sydney, thousands of Australians marched in solidarity in the city's central Hyde Park. One organizer said hatred, bigotry and racism are not only America's problems.
"We’re not marching as an anti-Trump movement per sé; we’re marching to protest the hate speech, the hateful rhetoric, the misogyny, the bigotry, the xenophobia and we want to present a united voice with women around the globe," organizer Mindy Freiband told Reuters.
Many of the protesters in Sydney and Melbourne wore pink hats that activists referred to as their "pussyhats".
The Washington march features a morning rally with a speaking lineup including a series of celebrities, Scarlett Johansson, America Ferrara, Amy Schumer, Frances McDormand and Zendaya, among them.
There are gatherings planned for Dublin and Galway this afternoon.
Organiser, Fanya O'Donoghue said people need to prepare for the next election in for four years' time:
Clashes with police
The march comes after more than 200 people were arrested in the US capital after activists clashed with police near the White House last night, throwing rocks and bottles.
Police responded with tear gas, pepper spray and stun grenades.
According to reports from the Associated Press, a group of about 100 protesters smashed windows of downtown businesses about a mile from the National Mall.
Denouncing capitalism and Mr Trump, they targeted a Starbucks, a Bank of America and a McDonald's.
"They began to destroy property, throw objects at people, through windows,” the city's interim police chief, Peter Newsham told the AP. “A large percentage of this small group was armed with crowbars and hammers.”
He said six officers suffered minor injuries in the disturbance.
The confrontation escalated several hours after the President took his oath of office as the crowd of protesters swelled to more than 1,000 - some wearing gas masks and with arms chained together inside PVC pipe.
One protester said the crowds were "bringing in the cavalry."
When some crossed police lines, shouting, "Put the pigs in the ground," police charged with batons and pepper spray, as well as stun grenades, according to the AP.
It is unclear if any of the protesters will still be active for today's women's demonstration.