At about 10am on Saturday, as a quarter of a million or more people gather in protest at the base of Capitol Hill for the Women’s March on Washington, the newly minted President Donald Trump will be on his knees at “a service of prayer and reflection” at the National Cathedral.

From that perch four miles away from the White House he won’t, initially, be able to hear the civil rights legend Angela Davis, the feminist icon Gloria Steinem or the pro-choice campaigner Cecile Richards addressing the crowds at the march.

But the roar of the masses will surely reach the president’s ears later, as the speeches and A-list performers wrap up the three-to-four hour rally and the demonstrators march to the Ellipse, the large green space opposite the White House.

He may even see them if he steps out on to the balcony in the residence to admire the view of the Washington Monument.

“I hope he will be praying about how to be the president of all the people. We are not going to let our rights be rolled back by anyone,” Melanie Campbell, president of the National Coalition on Black Civic Participation and a speaker at the march, said.

The march organizers are still officially only expecting a crowd of 200,000 to 250,000 people – mostly women, but also many men – based on the number who have registered.

But those close to the planning acknowledge that they are prepared, in coordination with the police, city officials and thousands of volunteers and private security staff, if the numbers swell considerably, to half a million or beyond.

The event is expected to turn into one of the largest marches in US history and although it is not billed specifically as an anti-Trump protest, most of the causes represented are those deemed under threat from the new administration.

“I’m speaking out and marching because healthcare is under attack and women’s reproductive rights are under attack. So many awful things were said about women by the incoming president,” said Campbell.

“I put a lot of my energy under Obama into supporting black women’s leadership and women’s leadership overall – we could have broken the glass ceiling with Hillary Clinton and it didn’t happen, so that’s what’s motivated me to take part,” she added.

Campbell took exception to Trump’s gloomy inaugural speech and his portraiture of inner cities as being in a state of a crime-ridden “American carnage”.

“That’s not the urban America I know. Not that there are not problems, but it’s not all rocked with violence and poverty. America is already great, it’s a melting pot and must continue to be,” she said.

On Saturday morning, Campbell and other speakers’ voices will boom out from jumbotrons positioned among the thousands packing along Independence Avenue, calling for action on racial equality, climate change, criminal justice reform, a higher minimum wage, immigrants’ rights and a host of other progressive issues.

Among those on stage or seated next to the stage in VIP seating will be some of the 60-plus members of Congress from the Democratic party who were set to boycott Trump’s inauguration ceremony on Friday.

One of those is US representative Chellie Pingree. While Trump was being sworn in on Friday, she was visiting a Planned Parenthood center and a business owned by immigrants in her home state of Maine, before flying to Washington in the afternoon to get ready for the march.

On Saturday morning, she will gather with fellow politicians on stage at the rally and then march with some of the 5,000 women attending from the state of Maine.

“We need to do everything we can to let the incoming administration know we are not happy about their agenda. I’ve had unprecedented numbers of my constituents calling me worried about healthcare, the environment, public education, and they feel disrespected,” she said.

Pingree said Trump was the most unpredictable incoming president most people had ever witnessed.

While Trump struggled to get A-list names to perform at his inauguration celebrations, the Women’s March on Washington has attracted singing and acting stars such as Katy Perry; America Ferrera, who is also speaking at the rally; the Orange is the New Black cast member Uzo Aduba; the actor Scarlett Johansson; Cher and the young actor and singer Zendaya.

The march grew organically, from a feminist outcry on Facebook in the immediate aftershock of the election result. Organizers now hope it will kickstart a new era of grassroots activism across a long list of progressive issues chiefly related to equality and social and economic justice.

What began as an expression of frustration and a call to action on social media then morphed into a mass mobilization across multiple progressive causes. The event began to be physically realized on Friday as buses, trains and planes started pouring into Washington, packed full of women – and supportive male friends dotted among them.

Although so-called sister marches are taking place in as many as 300 cities across all 50 states, as well as internationally, thousands chose the capital city to make their presence felt next to the Capitol and the White House.

“This is what a plane full of women who are ready to resist the Trump agenda looks like,” read one tweet, showing a flight bound for DC packed with women smiling and waving ecstatically, some wearing pink knit “pussy hats” that have become an informal uniform for the march.

“I am a woman of color and a Muslim from an immigrant family whose country is now going to be led by someone who has been openly anti-immigrant and sexist, has attacked Muslims and is surrounding himself with some horrible people for his cabinet,” said Zahra Billoo, director of the San Francisco chapter of the Council on American-Islamic Relations and a speaker at the march.

“I don’t think that everyone who voted for Donald Trump is racist, but they were willing to overlook that part of his campaign for whatever reason, and that’s not acceptable. We’ve seen the fallout, in hate crimes, and there is a need for inspiration and community across racial lines, religious lines, family lines, across many communities in America right now,” she said.

Billoo said she found Trump’s inaugural speech “no better than so many of his campaign speeches”.

She saw in it coded language designed to further divide rather than unite voters and to erase civil liberties, she said.

“All of this ‘America first’ is one way of saying if you don’t agree with us, we will come after you,” she said. Following scuffles and arrests in protests coinciding with the inauguration on Friday, Billoo said she hoped the Women’s March on Washington itself would be a wholly peaceful rally and demonstration.

The march is due to finish at dusk. But Linda Sarsour, one of the four march co-chairs, hopes that the end of the event will mark the beginning of a new movement.

“Making the march happen has been one of the largest grassroots efforts that anyone has ever seen. This is a mass mobilization and we intend to hold the administration to account,” she said.