Thousands of Leave supporters 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, organized by UKIP and involving English Defense League founder Tommy Robinson.
Mr Robinson - whose real name is Stephen Yaxley-Lennon - has spoken to crowds, and UKIP leader Gerard Batten is due to speak later.
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 had been twice unable to get her Brexit deal through the Commons.
After Friday's result, some protesters outside Parliament shouted: Shame on you. But others cheered when the result was announced.
Mr Robinson, speaking to crowds, addressed the confusion over the latest defeat of the deal, and said: So Theresa May has lost her vote. Many people will be asking what does that even mean. It means we were betrayed. Today is supposed to be our Independence Day.
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.
David Coburn, a Scottish MEP who belongs to the newly established Brexit Party, praised the march, saying: Excellent, isn't it? It's like Cromwell's Army.
Some of the protesters wore hi-vis yellow vests, a copy of the symbol of France.
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. London Metropolitan Police said it had appropriate policing plans” in place.