Donald Trump Meets The Queen At Windsor Castle, Breaks Royal Protocol

US President Donald Trump has flouted protocol while meeting the Queen.

US President Donald Trump has met with the Queen at Windsor amid massive protests about his first official visit to the United Kingdom.

Queen Elizabeth II welcomed Trump at Windsor Castle on Friday local time as the US leader continues his whirlwind four-day visit to the UK.

Accompanied by his wife Melania, Trump shook hands with the Queen, then carried out an inspection of the Guard of Honour alongside the 92-year-old monarch.

Donald Trump and the Queen walk from the Quadrangle at Windsor Castle: Chris Jackson/Getty Images

Trump courted controversy during the brief encounter, failing to bow when he met the Queen and walking ahead of her while inspecting the troops -- both considered breaches of royal protocol.

Meanwhile, up to 100,000 people took to the streets in London, marching through the city centre to oppose the visit by the US President.

There was also a Women's March on London and one by the Stop Trump Coalition, with all protests converging on London's iconic Trafalgar Square.

The demonstrations even included a "Trump baby blimp" depicting the 72-year-old as a nasty-looking baby, which took flight next to Westminster Parliament.

A heavy police presence was on hand to control the big crowds, with only a few incidents reported, including a clash between Trump supporters and opponents outside a pub in Whitehall.

Earlier in the day, Trump met with the Prime Minister Theresa May at her country estate where the pair talked through a possible US-UK trade deal.

The bilateral catch-up comes after Trump gave an interview to London's Sun newspaper which caused controversy over his criticism of May's negotiating strategy on Brexit.

"I also want to thank Prime Minister May for pursuing fair and reciprocal trade with the United States. Once the Brexit process is concluded and, perhaps, the UK has left the EU, I don't know what they're doing to do but whatever you do is Ok with me. That's your decision," Trump told reporters, speaking alongside May in Buckinghamshire.

"Whatever you're going to do is Ok with us. Just make sure we can trade together. That's all that matters."

Trump's next stop is Scotland, the nation of his ancestors, where he's due to spend the weekend at one of his golf resorts.