Germany on Friday outlawed active support for the Islamic State, warning the "terrorist" group operating in Iraq and Syria also posed a threat to Europe.
Interior Minister Thomas de Maiziere said the immediate ban covered the recruitment, including on the Internet, of jihadist fighters, the use of Islamic State (IS) symbols and social media propaganda.
"Germany is a well-fortified democracy, there's no place here for a terrorist organisation which opposes the constitutional order as well as the notion of international understanding," he said in a statement.
The radical Islamist group, which has committed horrifying atrocities in the wide swathes of Iraq and Syria it controls, is also a public security threat in Germany, De Maiziere warned, adding: "We are resolutely confronting this threat today."
"Today's ban is directed solely against terrorists who abuse religion for their criminal goals."
The move, which was welcomed by conservative and centre-left MPs, covers all participation in the group on German soil, including via social media, at demonstrations, by trying to gather fighters or funding or displaying the black IS flag.
The Islamic State is also active in Germany in terms of "propaganda and agitating" on the Internet and "specifically courts supporters in the German language", the interior ministry said.
De Maiziere also reiterated concern over an estimated 400 German nationals who have travelled to Iraq and Syria to fight on the side of the jihadists, some of whom have returned to Germany.
"We must prevent radical Islamists bringing their jihad to our cities," he said.
The German government has ruled out taking part in US-led airstrikes against IS who, the US Central Intelligence Agency estimates, now has up to around 30,000 fighters in Iraq and Syria.
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But Berlin made a watershed decision last month to send arms to Iraqi Kurds battling IS jihadist militants.
Chancellor Angela Merkel defended the move to break with a post-war policy of refusing to send weapons into conflict zones by saying Europe's own security was at stake.
"The enormous suffering of many people cries to the heavens and our own security interests are threatened," Merkel said in a September 1 speech.
- 'Important signal' -
Stephan Mayer, of Merkel's Bavarian CSU allies, called the IS ban a "clear step", while Social Democrat Ralf Jaeger, interior minister of the western North Rhine-Westphalia state, said it sent an "important message".
Germany on Monday is due to put on trial a 20-year-old alleged jihadist suspected of being a member of IS, in the country's first court case involving the militant group.
The German ban comes ahead of a special United Nations session, expected to adopt a resolution demanding that countries adopt laws making it a serious crime to enlist as a foreign fighter for Islamist groups in Iraq and Syria.
US President Barack Obama is to chair the session on September 24.
German politicians and media Monday called for a crackdown on radical Islamist propaganda after ultra-conservative Salafists took to the streets of the western city of Wuppertal, calling themselves the "Sharia Police".
Obama this week vowed to expand an offensive against IS extremists, foreseeing new air strikes against IS in Syria, expanded attacks in Iraq and new support for Iraqi government forces.
The IS has declared a "caliphate" in parts of Syria and Iraq it controls and has been accused of widespread atrocities, including beheadings, crucifixions, rapes and selling women into slavery.