Jeremy Corbyn addressed an anti-Donald Trump on the second day of the president's state visit, launching an attack on far-right politics.
The Labour leader spoke to crowds from a stage in Whitehall as thousands of protesters took to London's streets on Tuesday, gathering in Trafalgar Square and Parliament Square.
The president arrived in the UK on Monday, where he met with the Queen, senior members of the Royal and enjoyed a state banquet at Buckingham Palace.
Mr Corbyn refused to attend the banquet on Monday evening and said he stood "in solidarity with those he's attacked in America, around the world and in our own country".
Speaking to protesters, the Labour leader said: "In welcoming visitors from the United States, I hope there can be a conversation.
"I am not, absolutely not, refusing to meet anybody. I want to be able to have that dialogue to bring about the better and more peaceful world that we all want to live in."
However during a press conference with Prime Minister Theresa May, Mr Trump refused a request for a meeting with the Labour leader and dismissed reports of demonstrations as "fake news".
He said: "There were thousands of people cheering and then I heard there were protests. I don't see any protests. I did see a small protest today when I came, very small, so a lot of it is fake news."
Mr Trump described the Labour leader as a "somewhat negative force".
Mr Corbyn, who did not address the president by name during his speech, said the demonstrations in the capital showed how determined people are to "achieve a better place and a better world".
He added: "But I'm very disappointed, particularly today, on the wonderful festival of Eid, that our Mayor of London Sadiq Khan has been attacked as he has.
"I am proud that our city has a Muslim mayor, that we can chase down Islamophobia, anti-Semitism, any form of racism within our society because racism divides."
The Labour leader was referring to the ongoing feud between the London Mayor and Mr Trump, which was reignited when the president attacked Mr Khan, moments before landing in the UK, calling him a "stone cold loser."
The Trump Babysitters group estimated tens of thousands of protesters were involved in the demonstrations - fewer than an estimated 250,000 who gathered when Mr Trump visited the UK in July last year.
A 20ft orange inflatable of the nappy-clad president, made famous during Mr Trump's visit to the UK in 2018, took to the skies over Parliament Square.
A team of organisers wearing red jumpsuits and hats marked 'Trump Babysitters' launched the balloon to cheers from onlookers.
Shaista Aziz, from the Stop Trump coalition, said the blimp of the president clutching a mobile phone has "captured the world's imagination".
Roads were sealed off and police stood guard, as scuffles broke out between supporters of the president and protesters.
One Trump supporter was surrounded by an angry group of protesters shouting "Nazi" in Parliament Square.
Video footage posted on social media showed a milkshake was thrown at him before a scuffle broke out.
The man, who said he was from London but did not want to be named, said he was "angry" to have been targeted.
In another incident, an elderly pro-Trump supporter was pushed to the ground, as tensions flared on both sides of the demonstration.
Organisers of the Together Against Trump protest have billed the demonstration as a "carnival of resistance", declaring a "Trump-free zone".
A 16ft talking robot of Mr Trump sitting on a gold toilet, which says the phrases "No collusion", "You are fake news" and "I'm a very stable genius" - the audio of which is Mr Trump's own voice - attracted onlookers.
Another was selling toilet paper with Mr Trump's face printed on it, James O'Brien, shouted at Trafalgar Square: "The only toilet paper in the world that's already got crap on it."
Supporters of Mr Trump also took to the streets to back him.
Lewis Metcalfe, 28, from Richmond in North Yorkshire, said he took a day off work to travel to London and offer "a difference of opinion".
"I'm obviously going to be a minority today", said Mr Metcalfe, who was at Parliament Square wearing a Make America Great Again cap.
Scotland Yard's Deputy Commissioner Sir Steve House said the force is not in a position to estimate how much the operation will cost but added the US President's last visit to the capital cost the Met about £2.9 million.