Donald Trump backtracked on his combative remarks about Britain on Friday as he called the relationship between the two countries "the highest level of special".

The US President and wife Melania met with the Queen at Windsor after spending the morning with prime minister Theresa May, but the visit was marred by huge protests over Trump's presence.

The President joined May in an awkward joint press conference after slamming her Brexit plan and warning a US-UK trade deal could be off the table in an astonishing interview on Thursday.

Britain's Queen Elizabeth II, centre, stands with US President Donald Trump and first lady Melania on the dias in the Quadrangle as they listen to guardsmen play the US national anthem. Photo / AP
Britain's Queen Elizabeth II, centre, stands with US President Donald Trump and first lady Melania on the dias in the Quadrangle as they listen to guardsmen play the US national anthem. Photo / AP

Speaking after their meeting a day later, Trump called May an "incredible woman" who was "doing a fantastic job" and insisted relations "have never been stronger".

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But the President repeated his observation that Boris Johnson would be a great prime minister.

Trump and May also clashed on immigration in the news conference, with the President saying it had been "very bad" for Europe and was changing the continent's culture. The prime minister said the UK had a "proud history" of welcoming people to the country but that it was important to have a "set of rules".

The President and First Lady then joined the Queen and Prince Philip at Windsor Castle for a military display followed by a traditional tea.

U.S. President Donald Trump with Queen Elizabeth II, inspects the Guard of Honour at Windsor Castle in Windsor, England. Photo / AP
U.S. President Donald Trump with Queen Elizabeth II, inspects the Guard of Honour at Windsor Castle in Windsor, England. Photo / AP

Meanwhile, there were outbreaks of violence in London as demonstrators rallied in protests against Trump's policies, flying a "Trump baby" blimp over the Houses of Parliament.

The President had criticised May's handling of the Brexit negotiations in an extraordinary interview with The Sun on Thursday, warning that her strategy was not what people wanted — and suggesting it could scuttle a US-UK free trade deal.

But on Friday, the two leaders said they hoped to pursue a bilateral trade agreement, with Trump saying he wanted to ensure the US "can trade and we don't have any restrictions" on commerce with the UK.

"I have a lot of respect for the prime minister," said Trump, adding that the News Corp title had not printed the "tremendous things" he said about May. "I think she's a terrific woman."

President Donald Trump and British Prime Minister Theresa May hold hands at the conclusion of their joint news conference at Chequers, in Buckinghamshire, England. Photo / AP
President Donald Trump and British Prime Minister Theresa May hold hands at the conclusion of their joint news conference at Chequers, in Buckinghamshire, England. Photo / AP

It came as more than 60,000 people protested against him, with marchers in London on Friday waving "Dump Trump" placards as a six-metre Trump baby balloon flew above the city.

There were heated clashes between anti-Trump protesters near Trafalgar Square and those supporting jailed far-right activist Tommy Robinson. One man was dragged away by police after spitting at an officer, as both groups tried to drown out each other's chants.

The protesters headed down Regent Street towards Trafalgar Square for a massive rally, carrying signs reading, "You're not welcome here" and, "Ban guns not Muslims."

One popular chant was "He's going home" to the tune of Three Lions (Football's coming home), the soccer anthem that has been everywhere in England during the World Cup in recent weeks.

The streets were filled with people watching and shouting out their support. Those taking part in the march were a cross-section of society — young, old, parents and children.

A six-meter high cartoon baby blimp of U.S. President Donald Trump is flown as a protest against his visit, in Parliament Square in London. Photo / AP
A six-meter high cartoon baby blimp of U.S. President Donald Trump is flown as a protest against his visit, in Parliament Square in London. Photo / AP

The big balloon depicting Trump as an angry baby clutching a mobile phone was inflated and launched in front of a cheering crowd in the first act on a day of anti-Trump events in London.

The "Trump baby" was sent into the sky above Parliament Square in Westminster before it was grabbed by demonstrators leading the march, as the city put on a massive show of strength against the US President.

One speaker in Parliament Square mocked Trump, saying "Our crowd is bigger than his crowd."

Another shouted: "The only phobia you're allowed to have is Trump-phobia!"

Protesters displayed signs reading "Fight racism," "Trump stole all my bronzer" and "Feed him to the Corgis", and the roar from the marchers shouting and whistling was deafening.

Jon Spencer, of Kent, said he wouldn't have missed the protest. "I think history will record him as being very bad and I wanted to be able to say I did something," he told news.com.au.

He said his main issue was that Trump didn't seem to care about the damage he was inflicting. "He's a businessman and is all about the deal and the end result, but it's how you get there and the effect it has on people he doesn't seem interested in."

A six-meter high cartoon baby blimp of U.S. President Donald Trump is flown as a protest against his visit, in Parliament Square in London, England. Photo / AP
A six-meter high cartoon baby blimp of U.S. President Donald Trump is flown as a protest against his visit, in Parliament Square in London, England. Photo / AP

Trump met May at her country residence Chequers and is largely avoiding London, but US diplomats insisted it has nothing to do with the large-scale demonstrations.

However, he told The Sun he did feel "unwelcome" in the city. "I guess when they put out blimps to make me feel unwelcome, no reason for me to go to London," he said in an exclusive interview.

He said he used to love London, but wouldn't stay anywhere he was made to feel unwelcome.

"And when I say that I am talking about government, because the people of the UK agree with me."

He claimed to have received thousands of messages from Brits who supported him, and praised a west London pub for renaming itself the Trump Arms. "I love these people. They are my people," he said.

Trump has a long-running feud with London mayor Sadiq Khan, who he accuses of being soft on terrorism, and suggested the city leader had fuelled the unrest against him.

"I think he has not been hospitable to the government … he might not like the current president, but I represent the United States."

Hundreds of extra police have been deployed to protect the Trumps and a three-metre high steel fence — dubbed the "ring of steel" — has been set up around the US Ambassador's residence in Regent's Park, to safeguard against protesters including a group called Keep Trump Awake that aimed to keep Trump awake by making noise throughout the night.

The visit has led to the biggest mobilisation of officers since the 2011 London riots, with police called from all over the UK, at a cost of $18 million. Some forces have had to cancel hundreds of hours of rest days for staff so they can keep their streets safe while colleagues are in London protecting the President.

The US Embassy has even taken the extraordinary step of warning citizens to keep a "low profile" as the protests could "become violent"

Trump visited a British defence facility on Friday with Mrs May, before flying to Chequers for a working lunch.

TV comedy writer Lucy Guy, left, 41, with Emily Darnell, 40, hold signs during a protest in London. Photo / AP
TV comedy writer Lucy Guy, left, 41, with Emily Darnell, 40, hold signs during a protest in London. Photo / AP

A farmer whose land is along their flight path has allowed protesters to make a crop circle that spells out "F*** Trump".

From Chequers, Trump and the First Lady flew to Windsor Castle for an audience with the Queen.

They will then spend the weekend in Scotland — where police were also braced for protests — visiting his golf courses, before flying out on Sunday evening to Helsinki, Finland, for his eagerly awaited summit with Russia's Vladimir Putin.