At Westminster, crowds blocked traffic and some staged a sit-down protest in Parliament Square chanting "stop the coup" while others headed for Downing Street.
There were also impromptu demonstrations planned in Edinburgh, Cardiff, Manchester, Bristol, Cambridge and Durham after the Queen approved an order that will see Parliament suspended for more than a month.
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."
Two people carrying a "Brexit Now" banner were grappled as they passed the entrance into Parliament, prompting police to intervene.
Simon Hindmarsh, 47, a carpenter from West Yorkshire, was carrying one end of the large banner amid jeers and boos from the surrounding protesters.
He told the PA news agency: "We're leaving an undemocratic society so we can actually be a democratic one."
Mr Hindmarsh added they expected a hostile reception at the event.
"For the last 40 years, the EU has been the biggest scapegoat. I'm a leftie, we're not all Tommy Robinson supporters. I'm not just a thick racist northerner."
Several anti-Brexit protesters began to try to pull down the banner, prompting police to intervene in the crush close to the gates.