U.S. President Donald Trump, First Lady Melania Trump and Britain's Queen Elizabeth review items from the Royal Collection at Buckingham Palace, in London, Britain. Picture: Reuters/Carlos Barria

London - US President Donald Trump met Queen Elizabeth II at Buckingham Palace on Monday after he kicked off his state visit to Britain by branding the London mayor a "loser" and weighing in on the Brexit debate. 

With the sound of a 41-gun royal salute ringing across the lawn from nearby Green Park, the queen welcomed Trump and his wife Melania before they were treated to a military guard of honour.

The monarch later hosted a private lunch for the couple and showed them the royal art collection, ahead of a glittering state banquet in the evening.

But the day began with controversy as, even before his plane touched down, the president lambasted London Mayor Sadiq Khan, who has been highly critical of the red carpet welcome.

Trump called the mayor a "stone cold loser", adding: "Kahn reminds me very much of our very dumb and incompetent Mayor of NYC, (Bill) de Blasio, who has also done a terrible job - only half his height".

"In any event, I look forward to being a great friend to the United Kingdom."

Backing for Boris

Trump's three-day visit comes at a difficult time for Britain, with Prime Minister Theresa May due to step down within weeks over her handling of her country's exit from the European Union.

Trump weighed in on the divisive issue at the weekend, declaring that former foreign minister Boris Johnson would make an "excellent" new premier.

In a round of British newspaper interviews, he also recommended May's successor walk away from talks with Brussels, refuse to pay Britain's agreed divorce bill and leave the EU with no deal.

The UK-US "special relationship" was already under strain over different approaches to Iran, the use of Chinese technology in 5G networks, climate change, and Trump's personal politics.

Labour's Khan has led opposition to Trump's trip, writing a newspaper article on Sunday in which he compared the US leader to European dictators from the 1930s and 1940s.

"Donald Trump is just one of the most egregious examples of a growing global threat," he wrote.

His spokesman called Trump's tweets "childish" and "beneath the president of the United States".

Baby Trump blimp

Trump's first official visit to Britain last year was also marked by criticism of May's Brexit strategy and there were large protests.

Organisers are hoping for a repeat of the demonstrations this week and will once again fly a bright orange "baby Trump" inflatable.

Amnesty International on Monday unfurled a giant banner on the bridge near the US embassy reading: "Resist Racism, Resist Cruelty."

Labour leader Jeremy Corbyn said the protests against Trump were "an opportunity to stand in solidarity with those he's attacked in America, around the world and in our own country".

He and other opposition leaders are boycotting the state banquet, along with John Bercow, the speaker of parliament.

Trump also risked causing offence by describing Meghan Markle, the new US wife of the queen's grandson Prince Harry, as "nasty" for her previous criticism of him.

Harry was spotted with the US party at Buckingham Palace, as was Trump's daughter Ivanka, but new mother Markle is believed to have stayed away.

Special relationship

May and Trump are expected to emphasise the wider benefits of their old alliance when they hold talks with their teams at Downing Street on Tuesday.

On Wednesday, they will join other world leaders in the English port of Portsmouth to commemorate 75 years since the D-Day landings, which changed the course of World War II.

"Our relationship has underpinned our countries' security and prosperity for many years, and will continue to do so for generations to come," May said ahead of the visit.

May announced her resignation last month after failing to get her Brexit plan through parliament and twice delaying Britain's departure.

She will formally quit as her Conservative party's leader on Friday but will stay on as caretaker prime minister while her successor is chosen.

Three years after the referendum vote to leave the EU, Britain remains divided over its future.

Trump recommended the new government make an abrupt break with the EU if necessary, adding that there was "tremendous potential" for trade with his country.

Causing more potential embarrassment for May, Trump said he might also meet with Johnson and pro-Brexit leader Nigel Farage during his stay.

AFP