Anti-Islam group Pegida is responsible for the action, a spokesperson for the movement confirmed to NOS on Sunday afternoon.
The banner, placed at the As-Soennah Mosque, was accompanied by a mannequin wearing a beard, said Abdelhamid Taheri, an employee of the mosque, the NL Times reported.
According to Taheri, surveillance camera footage shows that the data-x-items were placed at around 5:15 a.m. on Sunday. At that time there were already people inside the mosque for the morning prayer, he said. Many people saw the banner and mannequin after morning prayer. "Then panic broke out. People were so shocked."
A spokesperson for Pegida said the group is responsible for the action, adding that Pegida is angry that requests to demonstrate at mosques in The Hague are always rejected.
"We are only allowed kilometers away. We do not accept that. We have the basic right to demonstrate, just like others have the basic right to exercise their faith."
The spokesperson said the banner and mannequin were placed in front of the mosque, not touching it. "Nothing was attached to the mosque. It is all on public road."
The Hague mayor Pauline Krikke took note of the incident with disgust. "In The Hague we have the right to disagree with each other, but within the framework of the law and with respect for each other's ideals, without unnecessarily hurting each other."
The police are investigating the incident.
Islam is the second-largest religion in the Netherlands, with many believers living in The Hague.