Trump hits back at London mayor on arrival in UK

On landing, he sent out a pair of tweets calling Khan a "stone cold loser" for criticising his red carpet treatment, which includes a private lunch and state banquet hosted by Queen Elizabeth II.

Khan, "who by all accounts has done a terrible job as Mayor of London, has been foolishly 'nasty' to the visiting president of the United States, by far the most important ally of the United Kingdom", he said.

"He is a stone cold loser who should focus on crime in London, not me," Trump wrote, likely referring to a spike in fatal stabbings in the capital which is causing widespread public concern.

The US leader also misspelled Khan's name, writing "Kahn".

The two have a long history of bad relations, occasionally clashing on Twitter.

Ratcheting up their feud a notch, Khan wrote a newspaper article on Sunday in which he compared the US leader to European dictators from the 1930s and 1940s.

Khan, a Muslim whose parents arrived in London from Pakistan and who became mayor in 2016, pointed to Trump's "deliberate use of xenophobia, racism and 'otherness' as an electoral tactic" as well as his travel ban efforts on mostly Muslim countries.

In a second tweet, Trump compared Khan to New York York's liberal Mayor Bill de Blasio, with whom he has also clashed, writing that the difference between them was that London's leader was "only half his height".

Khan's spokesman responded that "childish insults" should be "beneath the president of the United States".

London is bracing for two major protests on Monday and Tuesday, with organisers planning to fly a blimp of a baby Trump dressed in a diaper, with an angry expression on his face.

Several British opposition leaders and parliament speaker John Bercow have also turned down invitations to attend Monday's banquet with the queen in protests at Trump policies and behaviour.