(CNN) - For 29-year-old communications worker Naomi Penfold, taking the train down to London from Cambridge for Saturday's "Put It to the People" rally was daunting.
"It's been over 10 years since I have been on a march," she said. "Normally, we Brits stay quiet and grumble. But with Brexit there is too much at stake."
Armed with a homemade sign saying, "I'm British and I'm bothered," Penfold said she wants Brexit overturned, as smoothly as possible and as soon as possible, like many of the people who took part in London's biggest protests since the Iraq war.
More than a million participated in the marches, organizers said. CNN has not independently verified the figure.
Like many others, Penfold added her name to the 4.7 million already on a petition on Parliament's website before setting out for London, urging UK lawmakers to revoke Article 50 -- the legal process for leaving the European Union -- and cancel Brexit altogether.
But the question that many attendees of Saturday's marches asked themselves was this: Will any of it make a difference?
The United Kingdom was set to leave the European Union at the end of this month, but Brussels offered Prime Minister Theresa May a lifeline this week when it agreed to a delay in order to avoid a no-deal Brexit that would have occurred March 29.
The EU agreed to delay Brexit until May 22 if May gets her withdrawal agreement passed by Parliament next week.
But if May's deal fails with MPs -- again -- EU leaders said the UK could stay only until April 12, at which point they would expect the UK to present a clear plan about what it wants to do next.
But despite the petition to revoke Article 50, May has made it clear she feels that would be undemocratic. And she's in no mood for a second referendum on Brexit.
Penfold, like other marchers who spoke to CNN, questioned how May could put her Brexit deal with Brussels up for repeated votes before Parliament, while the public was given only one referendum to make their voices heard.
The marchers had plenty of concerns.
Nico Hall, who is half-Portuguese, traveled to London from Sheffield with his 4 1/2 year old rescue greyhound Ginnie. He said he's worried about her pet passport because he takes her abroad a lot.
"She's after a Wooferendum," he said jokingly, referring to a makeshift coat he had fashioned for her out of sheets of paper that said the same.
Briton Barry Englefield and his German wife, Waltraud, traveled 140 miles to London from Monmouth in South Wales for the march.
"There is no us and them between the UK and the EU," Waltraud said. "There is only all of us. Europeans."
She is one of the 3 million EU citizens who doesn't know whether they can stay after Brexit and hasn't yet applied for permission to stay after the UK leaves the bloc. "Brexit will never break us apart," said Barry.
But Penfold said Saturday's march was about displaying how many Brits were opposed to leaving the bloc, Penfold said.
"It's about showing the rest of the world," she told CNN. "This may not make a difference in (Parliament), but I want everyone to know how many in the UK want in, not out. And we are united in that."