“Death to the Jews” chanted the crowd waving the black flags of the Islamic State, or ISIS as it used to be known. They were looking for new supporters for their cause, the creation of a worldwide caliphate answering to the man who now calls himself Ibrahim: a zealot too radical even for Al Qaeda who has stormed through Syria and Iraq carrying out mass executions, crucifying rivals, beheading enemies. But these marchers were not in Syria or Iraq; they were in The Hague in The Netherlands. And their message was one tailored to the disaffected young descendants of Muslim immigrants in Europe.

“We are Moroccans,” went out the cry over a portable loudspeaker. "The French killed the Moroccans but they didn’t kill them all; the grandchildren of the few men left protest against the West, America and the Jews.”

Many of the demonstrators covered their faces with Palestinian scarves or balaclavas. “Anyone who doesn’t jump is a Jew,” someone shouted as the whole group started jumping in a scene that might have been ludicrous if it weren’t for the hateful message. “Death to the Jews!” the crowd shouted in Arabic.

This scene last Thursday came in the wake of an earlier demonstration supposed to defend the Palestinians suffering in Gaza, which turned quickly into a hatefest targeting Israel, with people carrying placards that screamed “Zionism is Nazism.” But while the comingling of pro-Palestinian, anti-Zionist and anti-Semitic sentiment has become all too common in European protests in recent weeks, that the battle flag of the Islamic State waved in the streets of The Netherlands on July 24 is something new and particularly dangerous.

All rallies in Dutch municipalities require permission from the local city council, the police and the public prosecutor’s office. The ISIS demo had been granted permission on the grounds that it was in support of the detained Dutch recruiter for jihad, Oussama Abu Yazeed. But the fact that the mayor’s office in The Hague either was unaware the rally was ISIS-linked or deemed it legitimate regardless has raised serious questions about the city council’s judgment.

Dutch Labor Party (PVDA) member Ahmed Marcouch, a former policeman who sits on the parliament’s security and justice committee, was one of the many who criticized the local government: “Unacceptable!” he tweeted. “Threatening journalists and shouting racist statements is punishable by law.”

Marcouch, who has Moroccan roots himself, wants the Muslim community in The Netherlands to be more vigilant in opposition to ISIS and similar groups. The footage shot at the protest clearly shows a number of very young boys: “What are these kids doing there in the first place?” he asks. “ISIS is pure barbarism, it is bloodthirsty,” Marchouch told The Daily Beast in an interview. “We can’t allow them to win our children away from us.”

Muslims often are criticized for not speaking out strongly enough against the retrograde radicalism of violent jihadists. But Marcouch does not mince his words: “The greatest insult of ISIS may even be toward the Muslims and Islam itself,” he tells us. “I call on the Muslim community: Stand up and don’t allow your religion to be hijacked by these idiots. Don’t make light of them, but make yourself strong against them, these barbaric criminals. Muslims have to speak out: ‘Not in my name! Stay away from my faith!’”

Marcouch has been arguing inside the parliament and out for more fieldworkers from within the community to prevent what he calls “religious derailments”: “You can only prevent this from happening by offering an alternative theological concept,” he says. “The parents play a part in this, too, in how they educate their kids. What’s the matter with you, allowing your kids to run with this lot? The community is much too silent. ‘We’ stand for civilization and modernity and everyone who wants to be a part of that. ‘They’ are those who reject democracy and even use violence. You have to define your opponent sharply.”

Marcouch’s decade serving with the police has taught him the limitations of government institutions. “We are a democracy so we can’t just lock people up whenever we feel like it,” he says. “That’s why expertise is paramount; that’s how you expose them.”

It is not only the people of The Netherlands who are worried. In a letter to Dutch Prime Minister Mark Rutte, Shimon Samuels of the Simon Wiesenthal Center wrote about the concerns of the wider Jewish community when it sees these racist demonstrations espousing the cause of the so-called “Islamic State” that opposes everything about democracy.

Whether the city of The Hague believes racism will simply ebb away when ignored, or just does not take the threat of ISIS seriously enough, is unclear. But ignoring the threat coming from gatherings like this aimed at legitimizing the warmongering of a growing group of fanatical and extremely violent radicals is ill advised. They are trying to win the hearts and minds and expendable bodies of young people, persuading them to go fight alongside the hundreds of Europeans who’ve already joined the gruesome war in Iraq and Syria.

The Hague’s Mayor Jozias van Aartsen recently claimed on Dutch radio that no red lines were crossed by the protests, but such declarations are facing mounting incredulity when pictures circulate on the Web like one posted by an Iraqi-Dutch citizen in the IS-ruled Syrian city of Raqqa. It showed him surrounded by the severed heads of seven men spiked on an iron fence. The photograph looks as if some parts of it may have been faked, but the sentiment is genuine enough. Beheadings, for ISIS, have become a kind of sport.

Many of the young people who end up surviving this horrific war for the caliphate will haunt Europe when they come back home, and security services all over the continent and, indeed, in the United States, are concerned. On Friday the Dutch public prosecutor's office finally announced that an investigation into ISIS in The Netherlands is underway.

One thing is certain, ignoring ISIS will not make it disappear and reasoning with people who want to spike the heads of their adversaries on fences should not be an option.