Tens of thousands of people have filled the streets of London to protest Donald Trump's first visit to the UK.

An estimated 100,000 people marched through the British capital under sunny skies to make their voices heard.

Earlier on Friday, a balloon depicting Mr Trump as a snarling baby took flight for two hours in Parliament Square right next to Westminster.

Nearly $20,000 was raised via crowdfunding to get the six-metre-tall inflatable into the air in what kicked off a day of protest.

Other protests included a Women's March on London and one organised by the Stop Trump Coalition, with both forming to meet up in London's iconic Trafalgar Square.

Anti-Trump protester Elizabeth Sharma said she was marching because she was "fed up" with the way Mr Trump was trying to manipulate the world to fit his views.

Sorry, this video has expired Trump baby blimp lifts off over London

"He thinks he is right on everything," she said.

"He has no understanding of the science of climate change, he is going to destroy the planet, he divides people.

"I mean, I could go on for hours."

Outside of London, similar protests were held across Britain in cities such as Manchester, Belfast and Glasgow.

Sorry, this video has expired UK opposition leader Jeremy Corbyn leads anti-Trump protest

Huge police numbers were on hand to manage the crowds in London, with only small flare-ups occurring when the demonstrators clashed with pro-Trump counter-protesters outside a pub on Whitehall.

Wearing Make America Great Again hats, the small group chanted "USA" and also voiced their support for English far-right activist Tommy Robinson.

One Trump supporter, who gave his name as Saul Goodman (the same name as a fictional lawyer in a TV series Breaking Bad), said he made the 350-kilometre trek down from Manchester solo to "welcome" Mr Trump.

'Saul Goodman' says Donald Trump puts his country first, as all leaders should. ( ABC News: Jack Hawke )

"I have a lot of respect for the man, he's the kind of person America, and the West in general, needs these days," the 30-year-old said.

"He's standing up to the people that need to be stood up to, and he's putting his country first, as all leaders should be."

Michael, who did not want to provide his surname, happened across the march and was given a MAGA hat by an American friend.

Michael says he likes Mr Trump because of his policies. ( ABC News: Jack Hawke )

"To say that he [Mr Trump] shouldn't visit other countries or that he's not welcome when he represents one of the biggest superpowers in the world seems crazy to me," he said.

The 32-year-old Londoner said he liked Mr Trump because of his policies.

"I think a country like America should attempt to be exceptional and build exceptional companies and establish global leadership," he said.

"It has the possibility to achieve a lot and I think our country has kind of lost its way — I don't even know what the point of this country is anymore."

Mr Trump is expected to face more protests in Scotland as he embarks on a two-day trip to his Tunberry golf course.

But supporters of Mr Trump will be out in force in London on Saturday, with a rally planned at the new US Embassy in London with speakers from UKIP, the Football Lads Alliance and For Britain.