By Laura Smith-Spark CNN

(CNN) -- Hundreds of thousands of people in the United States and around the world are joining marches Saturday to raise awareness of women's rights and other civil rights they fear could be under threat under Donald Trump's presidency.

The key focus of the day is the Women's March on Washington, which organizers say could attract a quarter of a million participants.

The march, which began with a modest Facebook call in the aftermath of the November election, has grown into what could be one of the larger political demonstrations ever seen in the US capital.

But there are also more than 600 "sister marches" planned around the United States, with some of the biggest expected in Boston, New York, Chicago and Los Angeles.

And women and men in cities around the world -- including Sydney, Berlin, London, Paris and Cape Town, South Africa -- are also marching in solidarity and in opposition to the values they think Trump represents.

'Girl Power vs. Trump Tower'

Australia was the scene of the first major international march, with thousands joining an anti-Trump protest in downtown Sydney.

Organizers said up to 5,000 people attended the protest at Martin Place; police estimated the number was closer to 3,000.

Chants from the crowd included "women united will never be defeated" and "when women's rights are under attack, what do we do, stand up, fight back." Some carried banners with messages such as "Girl Power vs. Trump Tower" and "Dump the Trump."

A separate group of about 30 Trump supporters also held a rally in Sydney. The police physically restrained some of them, blocking them from entering the same area as the anti-Trump protest group.

Protest organizers in New Zealand's capital, Wellington, said about 700 people turned out there for a women's march. Marches were also held in Auckland, Christchurch and Dunedin.

Protesters also joined together to march in Nairobi, Kenya -- the African nation that was home to former President Barack Obama's father.

Marchers in Cape Town carried banners with slogans such as "climate change is a women's issue" and "so over mediocre men running things."

Europe marches

Big crowds turned out Saturday in dozens of cities across Europe, with marchers including men, women and children.

Protesters who gathered outside one of Rome's most famous structures, the Pantheon, on Saturday morning carried signs such as "Yes we must" and "Women's rights are human rights."

Demonstrators also took to the streets of Berlin, Frankfurt, Munich and other cities in Germany.

Katy Rea, who was at the march in Berlin, told CNN: "There are around 1,000 people. Lots of families, children. Very friendly atmosphere. Some police are present, but it's tame and relaxed."

Marches were also planned in cities up and down the United Kingdom, from London to Cardiff, Liverpool, Belfast and Edinburgh.

In London, thousands were expected to join a march starting outside the US Embassy and ending with a rally in the city's historic Trafalgar Square.

Marcher Victoria Dawson told CNN the atmosphere was "positive, inclusive, electric."

London Mayor Sadiq Khan -- who before the US presidential election told CNN that Trump's views of Islam were "ignorant" -- posted a tweet encouraging Londoners to join the march and "show how much we value the rights every woman should have."

On Friday night, a crowd of predominantly female protesters gathered in Brussels, Belgium, to denounce sexism and protest against Trump.

'Sea of pink' planned

The Women's March on Washington, which begins at 10 a.m. ET near Capitol Hill, comes on the heels of a slew of protests there on Inauguration Day.

Many of those taking part are concerned about Trump's agenda, his past remarks that appeared to demean women and allegations against him of sexual misconduct -- which he has denied.

Thousands of people have also been busy making cat-eared knit hats to wear at the march, with the aim of creating a "sea of pink" on the National Mall.

Organizers of the so-called Pussyhat Project said they have received tens of thousands of handmade hats to distribute to marchers, with submissions flooding in from all 50 states and as far away as France and New Zealand.

CNN's Rebecca Wright and Alison Daye contributed to this report.

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