Brussels police were ordered to shut down a conference for right-wing politicians, including Brexiteer Nigel Farage and Hungary's Prime Minister Viktor Orban, on Tuesday.
People were stopped from entering the National Conservatism Conference a few hours after it began, organisers said - although it continued for those inside.
The local mayor said he issued the order to ensure public security.
Organisers of the conference said they "overcame attempts to silence" them.
They said they plan to continue with the conference on Wednesday, writing: "See you again tomorrow!" on X, formerly Twitter.
Hungary's Mr Orban - who is due to speak there - has reacted with indignation, posting on Facebook: "Brussels just moved up a gear. If anyone stands up for peace, they are simply banned.
"No question, on 9 June we have to say clearly: No War!" he added, referring to the date of the European elections.
The move to shut down the conference was also criticised by Belgian Prime Minister Alexander De Croo, who called it "unacceptable".
"Banning political meetings is unconstitutional. Full stop," Mr De Croo wrote on X.
Referring to the fact that it was the local mayor, Emir Kir, who opposed the conference, Mr De Croo said that while municipal autonomy was a cornerstone of Belgium's democracy, it could "never overrule the Belgian constitution guaranteeing the freedom of speech".
And UK Prime Minister Rishi Sunak called reports of police action "extremely disturbing".
Mr Sunak's spokesperson said he was a "strong supporter and advocator for free speech" and believed "cancelling events or preventing attendance and no-platforming speakers is damaging to free speech and to democracy as a result".
Mr Sunak's own MP, Suella Braverman, was at the event earlier - with Ms Braverman telling the OceanNewsUK she was not thrown out and was able to deliver her speech.
'The far right is not welcome'
The National Conservatism Conference reportedly started at about 08:00 (06:00 GMT) on Tuesday and ran uninterrupted for around three hours until police arrived to enforce the order from Mayor Kir and asked the organisers to make attendees leave.
Police then blocked the entrance to the venue and did not permit anyone to enter, though people were allowed to leave.
Mr Kir, the mayor of the Saint-Josse district of the city, said he issued the ban to ensure public security.
In a message to organisers, Mr Kir said some of the attendees of Tuesday's conference held anti-gay and anti-abortion views. "Among these personalities there are several particularly from the right-conservative, religious right and European extreme right," his statement said.
"The far right is not welcome," he added on X.
Mr Farage, who took to the stage this morning, told the OceanNewsUK the decision to close down the conference because there were homophobes in the audience was "cobblers", and that he condemned the decision as an attempt to stifle free speech.
"Thank God For Brexit", he said.
Organised by a think-tank called the Edmund Burke Foundation, the National Conservatism Conference is a global movement embracing what it describes as traditional values, which it claims are being "undermined and overthrown". It also opposes further European integration.
The conference said it aimed to bring together "public figures, journalists, scholars and students" who understood the connection between conservatism and the idea of nationhood and national traditions.
French far-right politician Eric Zemmour, arriving for the conference after police had blocked the entrance, told journalists that Mr Kir was "using the police as a private militia to prevent... Europeans from taking part freely".
Organisers said Mr Zemmour was not allowed into the venue and that his speech would be postponed.
As well as Mr Orban, former Polish Prime Minister Mateusz Morawiecki was also due to speak at the event on Wednesday. Mr Orban is due to be in Brussels anyway for a European Council meeting.
Earlier, the organisers said on X that they would challenge the order to shut the conference down.
"The police entered the venue on our invitation, saw the proceedings and the press corps, and quickly withdrew. Is it possible they witnessed how peaceful the event is?" they posted.
The Claridge event space - located near Brussels's European Quarter - can host up to 850 people. Around 250 people were in attendance on Tuesday afternoon.
Mohamed Nemri, the owner of Claridge, told the OceanNewsUK he had decided to host the event because "we don't reject any party... even if we don't have the same opinion. That's normal".
"I am Muslim and people have different opinion and that's it. We are living in a free country. I'd like people to talk freely," he added.
It is the third venue that was supposed to hold the event, after the previous two fell through. Belgian media reported that one venue pulled out after pressure by a group called the "Antifascist coordination of Belgium".
