Sydney, Australia's central business district is in lockdown as police respond to a hostage-taking and potential terrorist incident which started Monday morning at a mall café.

At least 13 hostages are being held by at least one armed individual at a Lindt chocolate café located at Martin Place, a pedestrian mall located in the heart of Sydney, according to Australian media.

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Authorities reportedly responded to a call by a concerned citizen after noticing a suspicious-looking man with a large blue sports bag near the café. It was not clear if the hostage-taking started immediately as soon as the man with the bag entered the café, or later when the police arrived.

On live news television, at least three hostages can be seen with their hands pressed against the glass window of the café, apparently being used as a human shield. Later, a pair of hostages was seen briefly hoisting a black flag with Arabic writing against the glass window.

The black flag, similar to those used by ISIS groups in Syria and Iraq, raised speculations that the incident could be a terrorist attack by Islamic extremists. The police, in earlier reports, said they are still considering it "an armed incident".

Various sources estimate the number of hostages from 13 to as high as 50.

Offices around the area, including some of the country's biggest financial headquarters, were evacuated. The Sydney Opera House, though farther away towards the bay, was also reportedly evacuated as a precautionary measure. The police would later widen the scope of the lockdown and cordon off the main business district.

New South Wales police commissioner Andrew Scipione admitted in a news briefing more than three hours later to still having no communication with the "armed offender" inside the café (though he would later say when asked directly that there could be more hostage takers). The commissioner did not say what weapons the armed man or men are wielding.

Earlier, in another news conference, Prime Minister Tony Abbottappealed for calm and to not let the incident disrupt their lives.

 "I can think of almost nothing more distressing, more terrifying than to be caught up in such a situation and our hearts go out to these people," said Abbott.

Tagschocolate cafe, Hostage, lockdown, Terrorist, Lindt chocolate cafe