Police will patrol mosques across Sydney under an operation established to head off any reprisals from Thursday's anti-terror raids, but they insist it's for community protection and confidence.

There's "absolutely" no specific retribution threat police are worried about. However Superintendent Mark Walton said police might have to respond to "information that may arise".

A document, believed to be a threatening letter mentioning several Sydney mosques, is being investigated by detectives.

"There's a potential there that the crime of sending a threatening letter or using a carriage service to cause offence has occurred," Walton said.

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"Unfortunately I'm never amazed by the stupidity of some individuals ... when that stupidity moves into criminal conduct that's where we get very focused and deal with those people."

About 220 officers have been brought into Operation Hammerhead, a round-the-clock attempt to encourage people to continue with their usual lives.

"We work very closely with our contacts in the community to ensure that calm remains," Walton said of the high-visibility operation.

As part of Hammerhead additional police have been sent across Sydney.

"This is about making sure the whole community has confidence to go about their daily business."

Extra police from Hammerhead on Friday night patrolled the AFL and NRL games in Sydney and there weren't any incidents, Walton said.

Mounted units, dog squad and other police were at last night's NRL game.