Anti-Brexit marchers rally in Parliament Square
Thousands of protesters in favour of the UK staying in the European Union have marched in Westminster.
The People's March for Europe took a route through central London before a rally in Parliament Square.
Liberal Democrat leader Sir Vince Cable said there were a growing number of people worried about Brexit's impact.
The march came ahead of MPs voting on Monday on a bill that will overturn the act that took the UK into the EU and end the supremacy of EU law in the UK.
Remainers - many dressed in blue and yellow outfits and draped in EU flags - amassed outside Parliament on Saturday afternoon.
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Many carried "Exit from Brexit" placards or wore "Remoaner Till I Die" t-shirts.
Former Liberal Democrat cabinet minister Sir Ed Davey told marchers he had "gone from anger to distress, from fury to despair".
He added: "Since the Brexit negotiations begun there's a third emotion I've been feeling - embarrassment.
"Embarrassment at our country's leaders. Embarrassment for Great Britain."
Tory peer Baroness Patience Wheatcroft told demonstrators that Remainers needed to keep campaigning to stay in the EU.
She said: "We have to stop Brexit. Since we joined the EU we've had an unprecedented period of peace and prosperity. It must be right to try and maintain that.
"It's not undemocratic to try to persuade the electorate to think again about Brexit. That's democracy at work."
Organisers estimated there were between 10,000 and 15,000 people at the start of the march, adding that numbers rose to about 50,000 at its height as people joined along the way.
The police did not provide any estimates and the BBC is unable to verify these figures.
One marcher, wearing a blue beret emblazoned with yellow stars, told the BBC she had joined the rally because she felt "totally violated by the idea of Brexiting".
"I've lived, worked and loved in Europe for years. My whole existence has been a European existence," she said.
"My husband has a business in Europe. We worked for years to build this up. What's going to happen to that?"
One man, holding a home-made placard, said. "I don't believe people really knew what they were voting for.
"We keep being told those who voted to Remain have largely changed their minds but I don't believe that at all."
Sir Vince told the BBC growing numbers of people wanted the UK to keep its links with the European Union and this was the beginning of a "loud and powerful" movement.
"They (the government) are not listening - they've got tin ear," he said.
"They're making a complete mess of these negotiations - totally disunited, dysfunctional, a lack of preparation.
"Even if you believe in Brexit you must be in despair at the way they're approaching these negotiations."