The demonstration was called by a number of groups in response to the weekend VOX event. The protesters gathered under the slogan 'All to Colón against fascism'.
Many see the meeting in the Spanish capital as a significant step in the international consolidation of far-right ideology.
''Here it (fascism) is growing,'' one protester said. ''With (Javier) Milei's visit we are seeing the grouping of a lot of sectors of politicians and the business world, which is quite worrying and I think that has to raise the alarm a little."
Argentinian President Javier Milei is among the leaders attending Europa Viva 24, held two weeks before voters cast their ballots in the EU Parliament elections.
Milei is on the second day of a three-day visit to Spain. His itinerary does not include meetings with Spanish Prime Minister Pedro Sanchez, Spain's King Felipe VI, nor any other government official.
France's presidential candidate, Marine Le Pen, Portugal's Chega, Andre Ventura, and Chile's presidential runner-up, Jose Antonio Kast, also attended the event. Italian Prime Minister Giorgia Meloni appeared via video link, while Hungarian Prime Minister, Viktor Orban and former Polish Prime Minister, Mateusz Morawiecki pre-recorded their messages.
''Somehow, we have to go ahead and tell them that we are here and that all the rights we have achieved we are not going to allow them to take even one step back,'' another protester said.
Meanwhile, nearly 11,000 supporters - according to Vox - packed a former bull ring waving Spanish flags, as well as some from Argentina, Cuba, Venezuela and Israel.
They cheered speakers from Le Pen to Ventura, and jeered at every mention of Spanish Prime Minister Pedro Sanchez, the United Nations' 2030 Agenda, feminism and socialism.
Nationalism and sovereignty were at the forefront of the speeches, including one by VOX candidate for the EU elections, Jorge Buxade.
''On June 9 we decide that Europe is still in the hands of globalist fanatics who pursue the destruction of the countryside, the devastation of the middle classes, the unprotection of borders and the submission of Europe to the whims of a bureaucratic caste of UN rapporteurs. Or, on the contrary, we European nations regain control of our lives, our economy, and our freedom," he said.
French National Rally party leader, Marine Le Pen said ''The European Union wants to standardise the whole continent. While we want a Europe that guarantees the diversity of cultures and identities of the European peoples. The European Union wants to promote Islamism and the woke movement. Whereas we want a Europe of proud cultures and which allows each people to defend its own identity.''
Argentinian President, Javier Milei, spoke about the impacts of socialism on society.
"Since socialism is an ideology that goes directly against human nature, it necessarily leads to slavery or death. There is no other possible destiny. To open the door to socialism is to invite death," he said.
VOX leader, Santiago Abascal, acknowledged the growth of right-wing support.
"We are not alone. We are alive and we are very well accompanied by all of you and by magnificent international allies. Today we have been able to prove it once again and we have been able to hear voices here that form a chorus alongside ours from both sides of the Atlantic. A choir that proclaims that another world is possible," he said.
