Demonstrators gathered in central London and across the country for an anti-austerity march organised by The People's Assembly Against Austerity, which is calling for a general election.
Separately a group of pro-Brexit campaigners assembled outside St James Tube station for a protest on Saturday.
Leading campaigner James Goddard was reportedly arrested by police ahead of the event on a public order offence relating to incidents in Westminster earlier this week.
Mr Goddard's supporters were marching in London in yellow vests to demand Britain leaves the European Union.
The protesters marched on Westminster, later blocking Westminster Bridge as speakers addressed the crowd, according to reports on social media.
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Last week, it was reported that around 100 demonstrators took part in a similar protest in which they set off smoke bombs and stopped traffic around Parliament.
Austerity campaigners wearing yellow vests with slogans such as 'Britain is Broken' written on the back also marched in London today.
People taking part met outside the BBC's Broadcasting House before marching to Trafalgar Square later.
Temporary stalls lined the road up from Oxford Circus station, where organisations such as Stand Up to Racism and the Socialist Party were handing out leaflets and placards.
People carrying drums prepared to march and political songs were being sung and large trade union banners were being held up in the crowd.
National organiser Ramona McCartney said the protest was an attempt to "take back the political space taken up by Brexit".
Ms McCartney said protesters also wanted to show "solidarity with the left and working class in France by wearing the yellow vests today".
Protesters travelled over from France to join the march in the capital.
Laurie Martin, 26, and Erick Simon, 57, arrived from Normandy on Friday night.
Ms Martin said she came "to support the British because our demands are the same as those fighting austerity across Europe".
"All European countries must join up in this battle against austerity," added Mr Simon.
France has seen weeks of national anti-austerity demonstrations as part by the "gilets jaune" movement.
The march slowly made its way down Haymarket, ahead of speeches in Trafalgar Square by figures including Shadow Chancellor John McDonnell.
A man with a megaphone at the front of the march repeatedly led protestors in calls for a general election.
Addressing protesters in Trafalgar Square, Mr McDonnell said eight years of austerity was "tearing apart the very social fabric" of the country.
He told the crowd he expected MPs to defeat Prime Minister Theresa May's Brexit deal at Tuesday's vote.
Mr McDonnell said that then, when "the time is right", his party would move a motion of no confidence to "bring this government down".
"It's now here before us, we could have a socialist prime minister," he said.
Mr McDonnell read out a message from Labour leader Jeremy Corbyn to the crowd.
He said: "We need a general election now to bring about the fairer, more equal society we all want to live in and we stand ready to take power."
— Ed Crawford (@_edwardcrawford)January 12, 2019
Meanwhile pro-Brexit yellow vest campaigners demonstrated in Westminster for the second week.
A large group of protesters marched through Vauxhall in support of Britain's exit from the European Union.
Demonstrators were pictured blocking traffic next to Parliament Square and burning the EU flag.
It came after Mr Goddard was allegedly arrested at St James Park Tube station before 12pm on suspicion of a public order offence.
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It is understood the arrest is in connection with remainer MP Ms Soubry being harassed outside Parliament during live television interviews in College Green on Monday.
The Met said a man in his 30s had been arrested at St James Park tube station at 11.42am on Saturday on suspicion of a public order offence.
A spokeswoman said the arrest related to an incident which took place in Westminster on Monday January 7.
The man remains in custody at a central London police station, the force said.