Police say operations at the Sydney Opera House and in Manly have concluded and were sparked by posts on social media.

Officers descended on the Opera House this afternoon, turning back pedestrians and cyclists from the area.

Bars in the precinct were also closed as tourists gathered in nearby Circular Quay.

At the same time, an area around the Manly wharf was also cordoned off.

Police have turned people away from the precinct. (9NEWS)

People have been cleared from the area. (9NEWS)

Police have confirmed the operations were linked.

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

The 1.20pm ferry from Manly to Circular Quay was cancelled due to the operation.

It is understood the operation was sparked by a threat to the Manly ferry.

The usually bustling Opera House tourist hub was left eerily quiet, and temporary fences were erected to keep people back.

The usually bustling transport hub was left eerily quiet. (9NEWS)

A Sydney Opera House tour guide said she had been leading tourists around the building when police arrived.

"My tour was stopped mid tour... now I can't get back in," she told AAP.