Thousands of Leave supporters have gathered outside Parliament to protest against the delay to Brexit, on the day the UK had been due to leave the EU.
Traffic was brought to a standstill, amid chants of "Brexit now".
The March to Leave, which started in Sunderland a fortnight ago, has also arrived in Westminster.
A Make Brexit Happen rally is also being held, organised by UKIP and involving English Defence League founder Tommy Robinson.
Mr Robinson - whose real name is Stephen Yaxley-Lennon - is due to speak at the demonstration later, alongside UKIP leader Gerard Batten.
Meanwhile, a counter-protest by the group Stand Up to Racism is also planned for Friday afternoon.
It comes as MPs rejected Prime Minister Theresa May's withdrawal deal, which she negotiated with the EU, by 58 votes.
The UK had been due to leave the EU on 29 March, but both sides agreed to postpone the date last week after Mrs May was twice unable to get her Brexit deal through the Commons.
After Friday's result, protesters outside Parliament shouted: "Shame on you".
'Angry and frustrated'
The BBC's Sarah Walton, who is at the scene, said people have been meeting in different parts of central London and making their way to Parliament Square.
She said: "There are some pro-Remain campaign groups demonstrating in London today.
"But at the moment here, the majority are pro-Brexit supporters, some of whom have been telling me that they were always planning on coming to London today, 29 March, but for a celebration to celebrate the day the UK would leave the EU."
"People have been saying they have come here from all across the country. They are angry and frustrated. Some of the rally groups have said they plan to be here well into the evening."
A large stage has been set up for the UKIP rally, and a second stage has been set up by Leave Means Leave for the marchers - where Brexiteer Nigel Farage is due to speak later.
As the March to Leave travelled on its last leg from Chelsea to Westminster, rolling road closures were used to manage traffic.
Roads around Parliament Square were also closed due to the Brexit demonstrations.