Metal barriers were erected Thursday and onlookers kept about 150 meters (500 feet) away from the building, while police officers and security personnel guarded the area in a 90-minute lockdown.

"Following information on social media, police conducted an operation in the vicinity of the Opera House and Manly as a precautionary measure," a police statement said.

No further details were provided although Sydney's Daily Telegraph newspaper said the scare revolved around a threat received about an object inside the Opera House.

An Opera House spokeswoman refused comment.

Australia is on heightened alert for attacks by grassroots militants. Authorities claim they have thwarted a number of potential attacks, although there have been several "lone wolf" assaults.

jar/rc (Reuters, AFP, dpa)