Hundreds of potentially violent Islamic radicals, including at least 50 who have traveled to and from the Islamic State where they took up arms for ISIS, now live in Berlin, a top German law enforcement official told FoxNews.com Wednesday.

Investigators have identified 680 hardcore Islamists in the German capital, nearly half of whom are “geared toward violence,” said a spokeswoman for the Berlin office of the Federal Office for the Protection of the Constitution, Germany’s equivalent of the FBI. While the ISIS veterans are most worrisome, all pose a serious threat, she said.

“We consider the ideology of the 680 to not be compatible with our freedom and that can be dangerous.” — German law enforcement spokeswoman

“We consider the ideology of the 680 to not be compatible with our freedom and that can be dangerous,” Isabelle Kalbitzer told FoxNews.com, adding that the 50 estimated to have returned from Islamic State pose the most immediate threat.

The number of radical Islamists in Berlin has nearly doubled from the 350 identified in Berlin just five years ago. And at that time, fewer than 100 were considered violent. In recent years, as ISIS has risen, much of the Middle East has become engulfed in war and millions of refugees have poured into Europe, Berlin has become a magnet for terrorists, say experts.

According to a report in the newspaper Berliner Morgenpost, Berlin Integration Minister Dilek Kolat said Berlin is “stronghold of the Salafism,” referring to a virulent and violent strain of Sunni Islam that gave rise to groups such as Al Qaeda and ISIS. Bernd Palenda, head of the Berlin intelligence agency, concurred, telling the paper Berlin is “a hot spot for Salafists."

Throughout Germany, federal law enforcement estimates there are 7,000 active Salafists animated by the goal of a return to 7th-century Islamic society. They openly advocate stripping women of their rights and an austere way of life governed by a harsh form of Islamic Sharia law. They can be seen in Berlin handing out copies of the Koran and propaganda materials.

Now, the entrenched radicals have a new audience in the growing pool of Muslim migrants and refugees that have flocked to Berlin from the Middle East and northern Africa, she said. Nearly 50,000 refugees have landed in Berlin, most from Muslim-majority countries such as Syria, Afghanistan, Egypt and Iraq.

Recruiting by violent Islamists occurs in Berlin’s mosques and even in front of the city’s main refugee registration center. In addition, the Muslim Brotherhood, designated a terrorist organization by the UK as well as Egypt, has a recruitment presence at the Lageso refugee center in the heart of Berlin, said Kalbitzer said.

“There are meeting places where the Salafists are recruiting,” she added said. “It continues to happen in different circles.”

The poisonous message of firebrand imams can be heard on Fridays in the city’s mosques, where attendance is swollen by the influx of refugees. Last month, a cleric named Sheikh Hassan Shahrour delivered a sermon in which he praised child murderer and Hezbollah fighter Samir Kuntar. In a 1979 attack in Israel, Kuntar bludgeoned four-year-old Israeli girl Einat Haran, whose father and two other Israelis were also killed.

Kuntar was released by Israel in a 2008 prisoner exchange, and promptly went to fight in Syria, where Israel reportedly killed him in an airstrike in December.

“You should give him a big round of applause,” said Shahrour. The audience responded with clapping.

In 2014, the radical Danish Imam Sheikh Abu Bilal Ismail called for the extermination of Jews at a Berlin mosque. He said it was necessary to “destroy the Zionist Jews...,” and to “count them and kill them to the very last one. Don’t spare a single one of them....”

According to a 16-page court verdict obtained by FoxNews.com , he was convicted and fined the equivalent of $1,400 for incitement to hatred.