Pictures on Australian television show at least three people with their hands up against a window, and a black flag with Arabic writing.
Hundreds of armed police have sealed off Martin Place in Sydney’s central business district.
New South Wales police have asked people to avoid the area.
An Australian Broadcasting Corporation reporter said that gunfire had been heard at the scene, the Lindt chocolate cafe – but this has not been confirmed.
Major landmarks in the city such as the Sydney Opera House have been evacuated.
Local media report that the airspace over Sydney’s central business district has been closed
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