Diane Abbott, the shadow home secretary, has addressed a large crowd of protesters from a stage near Number 10.
"We are here outside 10 Downing Street trying to get Boris Johnson's attention, but let me tell you, before too long Jeremy Corbyn will be in 10 Downing Street and Boris will be gone," she said.
Some protesters demanded to know why the Labour leader himself was not at the rally.
Ms Abbott tried to rally those in front of her with the cry of "What do we want to do" - expecting a reply of "stop the coup" - but many shouted "Where is Jeremy?" instead.
The Labour MP added: "We cannot allow Boris Johnson to shut down Parliament and to shut down the voice of ordinary British people."
As she highlighted how the Tories "thought they could get away with it" and how the protesters are there to say they will not, there were sporadic cries of "how?" from those in crowd.
Protesters are also marching in a series of other cities today, including York, Bristol, Belfast, Birmingham, Manchester, Liverpool, Glasgow, Swansea, Leeds and Aberdeen.
Further mass demonstrations, organised by the People's Assembly Against Austerity, are planned to take place on Tuesday to coincide with MPs returning to Westminster.
John McDonnell, the shadow chancellor, urged Labour MPs to join the nationwide protest, calling Boris Johnson's decision to close parliament for up to five weeks an attempt "to shut down democracy".
"As elected Labour MPs across the country represent their constituents by joining in these protests, I urge other MPs to think of their constituents whose jobs and livelihoods will be put at risk in a no-deal Brexit," Mr McDonnell said.
"If Boris Johnson wants a mandate, then he should call a general election and put it to the people."
The shadow chancellor is due to address youth movements at the main London protest at Downing Street on Saturday.
More than 1,000 people have gathered in the centre of York to listen to speakers outside the Bettys tea rooms and The Ivy restaurant.
Many of those in St Helen's Square were carrying banners as well as EU and Yorkshire flags and hats.
Rachael Maskell - the Labour MP for York Central - was heckled during a speech by a lone Leave supporter from the centre of the crowd, who then argued with protesters around him.
But he left without further problems as a small number of police officers looked on.
Students and local residents have gathered in Oxford, to protest outside Boris Johnson's former university college.
"Being outside the very institution where he developed political profile with students at the college today denouncing him sends a powerful message to Johnson and the others leading this coup," said Lesley McKie, a 55-year-old Oxford resident.
John McDonnell, the shadow chancellor, has told a large crowd of protesters at Whitehall: "It is a fight to protect our democracy - we know what Boris Johnson is up to, it is not very subtle is it?
"He wants to close down our democracy and force through a no-deal Brexit."
Mr McDonnell said previous generations fought and made huge sacrifices, some giving their lives, in the fight for parliamentary democracy so they could "have their say over policies and the future of our country".
He added: "Boris Johnson, this is not about parliament versus the people, this is about you versus the people."
"More than 50 cross-party MPs have vowed to sit in an alternative House of Commons if Boris Johnson suspends parliament ahead of the Brexit deadline," reports Lizzy Buchan, The Independent's political correspondent.
"In an open letter, the opposition MPs branded the move an 'unconstitutional coup' and said parliamentarians must not allow the government to 'avoid scrutiny at this time of national crisis.'
"Tory MPs Guto Bebb and Antoinette Sandbach have signed the letter alongside Liberal Democrats leader Jo Swinson, Independent MP Heidi Allen, Green MP Caroline Lucas and representatives from the SNP, Plaid Cymru and The Independent Group for Change."
Although the biggest march appears to be in central London, people are out in force in the rest of the UK.
Protesters in Liverpool can be seen taking to the streets in the above clip.
And in Bristol traffic has been blocked by around 2,000 protesters, who marched to College Green and then walked around the city, through the middle of the busy Broadmead shopping area.
Lots of chanting could be heard in the city, including "What do we want? Democracy! When do we want it? Now!", "Hey, hey, ho, ho, Boris Johnson's got to go" and the ubiquitous "Boris Johnson, shame on you!"
All the marches have been dubbed "Stop the Coup!" protests by organisers.
Some protesters have asked why Jeremy Corbyn is not taking part in today's Stop the Coup marches.
It turns out the Labour leader is in Glasgow, for a three day trip to Scotland.
He was asked about comments made by Keir Starmer, about next week being the final opportunity to stop the UK from leaving the EU without an agreement.
"I'm sure he is correct because Keir has worked extremely hard as our shadow Brexit secretary," Mr Corbyn said.
"Yes, it is the chance and we will do absolutely everything we can to prevent a no-deal Brexit and the prime minister taking us into the hands of Donald Trump and a trade deal with the USA.
"That is the real agenda of the prime minister.
"There is a lot of work being done in preparation for next Tuesday."
Mr Corbyn also did an impression of Boris Johnson as he mocked the prime minister for lacking detail in his announcements.
Parliament reconvenes after the summer recess on Tuesday.
It is then expected to be suspended the following week.
Influential left-wing campaign group Momentum has called on its members to "occupy bridges and blockade roads" to protest Boris Johnson's decision to suspend parliament.
Here's the view from the stage which protest organisers have erected near Downing Street today:
Momentum / @JDCmoore
"The first speaker on the stage was a leave voter called James who said he 'voted to boost British sovereignty' and “believes that what Boris is doing is totally without a mandate” and that 'he did not vote for no deal'," a Momentum spokesperson said.
“Today hundreds of thousands of people in all parts of the country and across the political spectrum have come out to take emergency action against a blatant power grab by the prime minister," added Laura Parker, Momentum’s National Coordinator.
"We have a barely-elected millionaire prime minister who is happy to exploit a loophole in our flawed democracy to hoard more power at the top.Nobody voted for an Eton educated millionaire to hijack our democracy in his own self interest.
"He is part of the same tiny, privileged elite who has been hoarding power at the top and eroding our democracy for decades."
Protests will be held every day at 5.30pm across the UK for the foreseeable future, according to Another Europe is Possible, the anti-Brexit campaign group which organised today's rallies.
"We are not going away. We will be protesting EVERY DAY at 5:30 pm. Everywhere," the group said on Twitter.
The prime minister announced earlier this week that he planned to suspend parliament for almost five weeks this autumn, in an unprecedented move from Downing Street.
John McDonnell, the shadow chancellor and Diane Abbott, the shadow home secretary, both spoke to the Whitehall crowds from a stage erected near Number 10.
“It is a fight to protect our democracy – we know what Boris Johnson is up to, it is not very subtle is it?” Mr McDonnell said to the gathered protesters.
He added: “Boris Johnson, this is not about Parliament versus the people, this is about you versus the people.”
Jeremy Corbyn, the Labour Party leader, spoke to thousands of protesters demonstrating in Glasgow.
”I’m proud to be here with all of you supporting that, to say to Boris Johnson no way, it’s our parliament,” he said.
“No way do you take us out without a deal – we will stop you and give the people their rights and their say to determine their future.”
“Well think on Boris, it’s not on and we’re not having it.”
Protesters in London blocked traffic on Westminster Bridge as they demonstrated against the suspension.
“If you shut down our Parliament, we shut down your bridge,” they shouted.
Demonstrators also blocked traffic on Waterloo Bridge and near Trafalgar Square.
Organisers estimate that 100,000 people marched in London. At least three people were arrested in the capital, Scotland Yard confirmed.
A further 5,000 people marched in Bristol.
Another Europe is Possible is planning daily protests at 5.30pm, to be held every day for the foreseeable future.
Additional reporting by agencies
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