At a police training academy on the outskirts of Brussels, new recruits are wrestling one another to the ground - practising techniques of unarmed restraint. There are about 40 of them - fresh-faced young people in their 20s, men and women - but what's immediately noticeable is that with one exception, they're all white.
Watching them is Paul Jacobs. For 20 years he dealt with discrimination complaints in the Belgian police - and he's just finished a session teaching this class "inter-cultural communication".
They discussed why so many Belgian youngsters go to fight in Syria - a higher proportion, relative to the population, than from any other country in Europe. And a heated argument broke out when Suhaila, the only non-white recruit - from a Moroccan background, like many Belgian Muslims - said she could understand why young Muslims might become jihadis.
make it harder to accept diversity
"The whole class was reacting - over-reacting," Jacobs says. "It was the first time they had talked with someone of a Moroccan background."
For a visitor to Brussels, where more than a quarter of the population is Muslim, that's a surprising thought. But Paul Jacobs is not surprised.
"I am a little bit scared to use this term," he says. "But I think we live in a system of apartheid. You really have ghettos. And what is more important, and more dangerous, is not that people aren't living together - it's the mental ghetto."
You might think that Belgium, with its major two linguistic communities - Dutch-speakers and French-speaking Walloons - would find it easier than other countries to accept diversity. But Jacobs says the opposite's the case - because those groups are already so inward-looking.
"We always talk about the others in negative terms," he says. "We say: 'We are hard-working Flemish people, and the Walloons are lazy!' We think of 'us and them' - even between Belgians. And in the same way, we think: 'There are white people, the real Belgians - and newcomers.' Even though people from Morocco and Turkey have been here three generations… In the future this will put a very large question mark over how we can live together."
Graduate police recruits get a total of six hours' diversity training in a year-long course, he says, while non-graduate recruits get none at all.
And in Antwerp, a city where one in six of the population is Muslim, out of 2,600 police officers only 22 are non-white, Jacobs says.
The question of how Belgians of different backgrounds can live together has become all the more acute since the Paris attacks, and claims by the media that districts such as Molenbeek are hotbeds of Islamist extremism.
Students of one Brussels school, the Koninklijk Atheneum Anderlecht, where 80% of the pupils are Muslim, produced a video saying they wanted to enjoy life together, whatever their backgrounds.
The headmaster, Erik Van Den Bergshe, grew up in Molenbeek. "I really liked the inter-cultural environment, and I used to play basketball and football with all ethnic groups and religions," he says.
But since then many believe the character of Molenbeek has become more Islamic - and the opportunities for social mixing have declined.
I meet Sarah Turine, the local councillor in charge of youth policy, at one of the only cafes in the district that attracts people of different backgrounds. The recently opened Palais de Balkis has bright, modern design - whitewashed walls and stripped pine furniture - and offers the novelty of home-made halal charcuterie from lamb and beef.
"People in Brussels live side-by-side, but don't often meet one another," she says. She thinks divisions have been reinforced because many young Belgians of Moroccan and Turkish descent have reacted against anti-Muslim feeling since 9/11 by defiantly adopting a more religious identity.
"Young people want a Muslim identity," she says, "but they haven't read the Koran, so it's become a matter of slogans - that girls should wear hijab and boys should grow beards."
She echoes Paul Jacobs's view of a society divided by an unbridgeable gulf.
"Simplistic explanations are the most successful, especially the hardline talk from Saudi Arabia with its binary distinction between what is haram and what is not, between 'us' and 'them'."
These are the distinctions used by Islamist recruiters who deliberately target young people in districts like Molenbeek, she says.
Malika Saissi, a mother-of-four who organises social activities for women in Molenbeek, says: "For 20 years I've seen women who were afraid - afraid that their kids would fail at school, afraid they would get into drugs. But those women don't even care about all that any more. Today they're afraid of just one thing above all else - that their sons will become terrorists."
A group of concerned Muslim women - some with sons already in Syria - asked Belgium's Interior Minister, Jan Jambon, to join them shortly after the Paris attacks at a meeting in Molenbeek to discuss ways of preventing radicalisation.
But they say the minister failed to respond or to send a representative - and that he hasn't visited Molenbeek since the attacks.
They're disappointed that in his most-publicised comments about the terror threat, Jambon said he wanted to "clean up" Molenbeek - and would organise house-to-house searches there.
"The only thing I hear from the political leadership is repression - 'clean-up'," says the organiser of the women's meeting, Johan Leman, of Le Foyer community centre, a veteran campaigner for integration.
"If you say 'clean-up' to people who already don't trust you, what will their reaction be? What is lacking for now is a really good prevention programme where people who might understand the problem are invited for discussions - this is completely lacking."