Crowds demonstrated in Parliament Square on Wednesday evening, hours after the Queen approved the Prime Minister’s request to suspend Parliament.
One attendee, an NHS employee, said the crowd was moving towards Downing Street, estimating there were thousands on the march.
She said: “The atmosphere seems happy – everyone is getting on and there are lots of families.
“People are chanting “save our democracy” and “stop the coup” and some Extinction Rebellion protesters have climbed the trees.
“It’s peaceful at the moment, no one is being aggressive.”
The woman, who attended the march with her niece, had come to London from Shropshire for a meeting but decided to stay for the protest.
She added: “I’m a remainer, and I’m horrified that this has happened.
“I work in a diabetes service and I’m worried about my patients getting the insulin they need as most of it is imported from Denmark.
“Delays at ports could be a real issue for us and everyone in the NHS is doing our best but it is getting harder.
“I honestly don’t know what will happen next but I hope the government listens to this protest and that it could be the beginning of a snap election.
“They need to understand the level of feeling that people have. Some people who voted leave have changed their minds because they don’t like the direction this is going in.”
Shadow home secretary Diane Abbott told protesters outside Parliament: “At the end of the day, it doesn’t exactly matter where you stand on Brexit, it matters where you stand on Tory prime ministers closing Parliament because they don’t want to give people a say.
“If this was a Latin American country it would be called a coup, complete with American president publicly backing it. We have to stop this coup, not just for Parliament but for this country’s future and for our children’s future.”
Shadow chancellor John McDonnell told the protesters: “The message is simple, whatever side you’re on in the debate around Brexit, the message is absolutely simple here, we’re supposed to be a democracy, and that democracy is meant to be a parliamentary democracy.
“They have taken away the decision. We think the decision that Boris Johnson is frightened of is Parliament itself taking control of the agenda next week and they’re not allowing us the opportunity.
“By closing Parliament down, it effectively closes democracy down in this country.”
He began by telling demonstrators similar protests were under way in Manchester, with protesters using umbrellas as a nod to the demonstrations in Hong Kong.
“It is not too much to ask simply to allow Parliament to sit, debate and vote. I warn Boris Johnson this – you’ve unleashed a force which you do not understand. From that elite sense of entitlement, you do not understand the power of the people.”
The organisers said on Facebook: “Boris Johnson is trying to shut down our democracy to deliver on his Brexit agenda.
“This is disgraceful, but we can’t just rely on the courts or parliamentary process to save the day. We all have a duty to stand up and be counted.
“Join us tonight from 5.30pm on College Green by the George V statue to stand up for democracy and fight against Johnson’s coup. Bring and invite everyone.”