Now the Parliament wants the country's leaders to take over the sprawling complex that is just steps from the gleaming core of the European Union. It is the latest attempt to tighten security after radicalized Belgians emerged at the heart of terrorist attacks in Paris and Brussels in the last three years.
The sudden move against the mosque underscores the challenge for Western European leaders seeking to embrace what they call a "European Islam" that endorses pluralistic values. For too long, many officials say, they have stood by as imams preaching the ultraconservative interpretation of Islam favored by clerics in Saudi Arabia and Qatar have worked among their populations, encouraging the frustrated descendants of North African immigrants to wall themselves off from mainstream society.
But the very same crackdown on the mosque puts Belgian policymakers in the unusual position of picking and choosing among strains of Islam in the name of protecting freedom of religion and democracy. The dilemma has grown more pressing after Europe was struck repeatedly by Islamic State-inspired terror, often perpetrated by disgruntled citizens born inside the countries they have targeted.
The mosque's leaders "are trying to live in their splendid isolation with a radical point of view, and their aim is not to integrate into our society. And that is a big problem," said Servais Verherstraeten, one of the leaders of a Belgian parliamentary commission that recommended last month that the government break the Saudi government's 99-year rent-free lease on the mosque. The lease was handed to Saudi King Faisal in 1969 as a goodwill gesture by Belgian King Baudouin.
"We want in Belgium an Islam practiced by people who respect our constitution, who want to integrate into our country," Verherstraeten said. "There is the perception that there is something to hide in the most important mosque in the country."
Leaders of the mosque and community center, which is run by the Mecca-based World Muslim League, deny that they espouse a conservative vision of Islam and say that they are working to improve openness.
"I don't see any contradiction between what we're trying to do and European Islam," said Tamer Abou El Saod, the executive director of the Islamic and Cultural Center of Belgium, which oversees the mosque.
Abou El Saod, a polyglot Swedish businessman, swooped in to run the center at the end of May after his predecessor upset the parliamentary commission with halting testimony at a hearing. Several lawmakers publicly questioned what he was covering up. That former director was a replacement in 2012 for a director who was quietly asked to leave Belgium after authorities said he advocated radical ideology.
"We can admit that we had some internal management issues," said Abou El Saod, who described himself as someone who was sent in to fix problems. "This place has maybe not been communicating enough in Belgium."
"An imam who talks to people here has to be different from one in Oman," he said in an interview in his office at the mosque, where photographs of Mecca adorn the walls along with blurry likenesses of the old Belgian and Saudi kings.
The Saudi lease is unusual but not unique. The Saudi government operates an Islamic school near Washington Dulles International Airport, for example, and it helps fund mosques and imams around the world.
Belgian counterterrorism officials acknowledge that a move against the crowded mosque will do little to stem radicalization that more often comes over the internet or on the street, and they say they have no evidence that its imams have advocated violence or lawbreaking.
But they also say they were mistaken to adopt a live-and-let-live attitude to the squat, plain concrete mosque tucked in the corner of the central Brussels's Cinquantenaire Park, across the street from apartment buildings and the boxy office block that holds the E.U. diplomatic headquarters. On Fridays, worshipers spill from prayers and mix with joggers and suited bureaucrats taking strolls along the park's manicured paths.
In the half-century since the Saudi government took over the site, Belgian authorities say the mosque has espoused the hard-line interpretation of Islam favored in the conservative Gulf monarchy. That has undermined an effort originally intended to help serve Belgium's then-growing Moroccan and Turkish guest-worker communities, they say. The center is Belgium's main hub for conversions to Islam, and its religious and Arabic-language schools teach 850 pupils.
The Belgian move came at the same time the newly-named Saudi
crown prince, Mohammed bin Salman, announced in Riyadh that he wanted to fight extremist interpretations of Islam.
Belgium's migration agency is also trying to expel the mosque's top imam, an Egyptian cleric who is accused of preaching an ultraconservative vision of Islam to his flock during his 13 years in Belgium.
The reassessment came after the March 2016 attacks on the Brussels airport and subway that killed 32. Those attacks were led by Belgians with Moroccan roots. They are not believed to have ties to the mosque, but the violence disturbed Belgian security services and led to a broad rethinking of their strategy.
Although the Saudi government is the leading funder of the World Muslim Leader, which operates the mosque, the group is independent and more than 50 other Muslim nations also contribute to its operations. The league's top leader, Secretary General Muhammad Al-Issa, was Saudi justice minister until last year.
The mosque's longtime imam, Abdelhadi Sewif, said that he is mystified that the Belgian state is attempting to expel him and his family by refusing to renew their residence permits. He is appealing the decision, and joked that his trim beard was too short and his floor-scraping robe too long to satisfy radicals.
In an interview, Sewif said that he had always advocated a moderate form of Islam, and that he never called Muslims to cut themselves off from mainstream society, as Belgian authorities have accused him of doing. He said he was searching his past words for what had made him a target. Authorities have not let him see the evidence against him.
"When I tell Muslims that Muslims love their Muslim brothers and have to help each other, does this then mean you should hate the rest?" he said. "I have always fought violence and spoken up against it. . . . I have always preached about forgiveness in Islam and in behavior."
But despite the mosque's protests, the Belgian government appears ready to break the lease and boot out its imams.
Sewif "is a dangerous man to the national security of our country," Theo Francken, the Belgian state secretary for asylum and migration and an anti-immigration hard-liner, told the RTBF broadcaster.
Lawmakers said that they were taken aback by the testimony of the then-director and one of Sewif's imam colleagues earlier this year.
"The hearing was astonishing," said Gilles Vanden Burre, a member of the parliamentary commission. "It was not an open and progressive and tolerant Islam."
Inside Parliament, the two mosque officials outlined a vision of starkly regressive gender roles and a prickly attitude toward mainstream Belgian society, Vanden Burre said.
"It's not objectively proved that they have a link with jihad or radicalization. But all those processes to show their goodwill to integrate, they never did," he said.
Segregation leads to alienation, Belgian security officials said. And that can make people more receptive to jihadist messages.
Officials said there was no recent shift at the mosque that set off alarm bells to cause the new approach. Instead, several said, it was Belgium that had changed. Policymakers have hardened their attitudes in a challenging era of terrorism.
"Many of the guys say they are not violent, but then they actually do preach hatred," said a security official who is familiar with the investigation and spoke on the condition of anonymity to discuss internal intelligence matters. "The issue that we have in Belgium is that people are teaching an Islam that is incompatible with Belgium or Europe."
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)