Thousands of protesters carrying effigies of Donald Trump marched through Brussels after the US president arrived for talks with the EU and NATO.
"Trump not welcome" said banners waved by the crowd, which police said numbered around 9,000.
Organisers put the size of the demonstration at 12,000.
"He called Brussels a hellhole and yet he comes here like a conqueror," one protester told AFP.
Mr Trump sparked fury by deriding the Belgian capital as a "hellhole" ruined by Muslim immigration in January 2016 - two months before suicide bombers killed 32 people in the city.
The rally filled the central Bourse square of the city just hours after Mr Trump touched down in Air Force One for high-stakes talks with allies.
Mr Trump met Belgium's King Philippe and Queen Mathilde, then held talks with Prime Minister Charles Michel, telling him that fighting terrorism was a priority.
Security was tight throughout the city but the protest passed off peacefully.
Some wore t-shirts saying "Make Humanity Great Again" and "Donald Back to USSR" - referring to a probe into allegations of collusion between the Trump campaign and Russia.
Many carried blond-haired effigies of the US president, and there were also some of Mr Michel.
NATO is to join the US-led anti-Islamic State coalition, a diplomatic source said, meeting a key demand by President Trump that the alliance do more against Islamist terrorism.
"Member state ambassadors to NATO have just taken the decision," said the source who asked not to be named. "The decision still has to be formally endorsed by NATO leaders meeting tomorrow."
Earlier, Pope Francis urged President Trump to be a peacemaker at their highly anticipated first meeting, and Mr Trump promised he would not forget the pontiff's message.
The two men had never met before this morning but they have significant history having clashed publicly over subjects ranging from migration to unbridled capitalism and climate change.
They also disagree on issues like the death penalty and the arms trade but share a fervent opposition to abortion.
Pope Francis smiled faintly as he greeted Mr Trump outside the study and was not as gregarious as he sometimes is with visiting heads of state.
Mr Trump, seeming subdued, said "it is a great honour."
The two talked privately for about 30 minutes with translators.
During their highly anticipated first meeting, Pope Francis urged US President Donald Trump to be a peacemaker https://t.co/wFAQizmbaw pic.twitter.com/JGfxPHsBX6
— RTÉ News (@rtenews) May 24, 2017
At the end of the private meeting, the Pope gave Mr Trump a small sculptured olive tree and told him through the interpreter that it symbolised peace.
"It is my desire that you become an olive tree to construct peace," the Pope said.
Mr Trump responded: "We can use peace."
Pope Francis also gave Mr Trump a signed copy of his 2017 peace messag titled "Nonviolence - A Style of Politics for Peace," and a copy of his 2015 encyclical letter on the need to protect the environment from the effects of climate change.
"Well, I'll be reading them," the US President said. He also changed the background image of his twitter account to one of him sitting across from the Pontiff.
Honor of a lifetime to meet His Holiness Pope Francis. I leave the Vatican more determined than ever to pursue PEACE in our world. pic.twitter.com/JzJDy7pllI
— Donald J. Trump (@realDonaldTrump) May 24, 2017
Pope Francis was gifted a boxed set of five first edition books by Martin Luther King.
The visit came on the third leg of Mr Trump's first overseas trip as president, which has already taken him to Saudi Arabia and to Israel and the Palestinian territories.
Mr Trump was accompanied for the audience by his wife Melania and daughter Ivanka, both dressed all in black, in keeping with traditional protocol that is no longer always observed by all female dignitaries visiting the Vatican.
The audience took place in the private library of the Apostolic Palace, the official papal residence that Pope Francis does not use, having opted instead for modest lodgings in a Vatican guesthouse.
Immediately after the meeting, the 80-year-old pope held his weekly address to the faithful on St Peter's square while the Trumps were given a private tour of the Sistine Chapel and St Peter's Basilica.
