After a serene visit to the Vatican and with Pope Francis, US President Donald Trump yesterday arrived in Brussels as thousands of anti-Trump demonstrators took to the streets and the Belgian Prime Minister said he spoke with the President "without taboos".

Trump, who a year ago said Brussels had turned into a "hellhole", oozed charm as he met with Belgian King Philippe and moved on to a powwow with top ministers on the eve of a Nato summit, the main event of his two-day stopover in Belgium.

Prime Minister Charles Michel said he insisted that Trump should unequivocally back the Paris Agreement on climate change and honour the US commitment to multilateral trade agreements, two issues about which Trump has raised doubts.

"It was a discussion without taboos. There was no diplomatic language. The messages were direct from both sides," Michel said of their meeting.

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About a kilometre away, thousands of protesters chanted, "We don't want Trump! We don't want Trump!" They harangued Trump on everything from his plans to build a wall on the US-Mexican border to his treatment of women and promotion of the coal industry.

A day earlier Trump met Pope Francis at the Vatican.

The Pope and Trump exchanged sharp words last year but on Wednesday the Pope urged Trump to be a peacemaker and Trump promised he would not forget the Pontiff's message. They spoke privately for about 30 minutes with translators.

Trump finishes his trip in Sicily where he will attend a G7 meeting.

Climate change will take centre stage at the G7 meeting, and Trump has yet to clearly commit to respecting the landmark Paris Agreement aimed at controlling global warming.