'Suspicious man' arrested near Opera House

An 18-year-old teenager has been detained by police after he was spotted acting suspiciously outside the Sydney Opera House.

The backpack-carrying man was reportedly making extremist remarks when police officers and security in the Opera House forecourt approached him on Thursday morning.

The man is being questioned by police and it's believed he suffers from mental health issues.

The incident comes two days after the Islamic State group issued a directive to its "lone wolf" followers to stab, shoot, poison and run over Australians at iconic attractions such as the Opera House.

"Kill them on the streets of Brunswick, Broadmeadows, Bankstown, and Bondi. Kill them at the MCG, the SCG, the Opera House, and even in their backyards," the first issue of the terrorist group's new online propaganda magazine said.