The northern hemisphere woke to the shocking scenes as TV networks ran continuous live broadcasts from Martin Place but world leaders through their embassies had already been briefed.

Britain’s Prime Minister David Cameron rang the office of his Australian counterpart Tony Abbott as the events unfolded overnight GMT for a briefing and to offer anything his government could do.

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“It’s deeply concerning and my thoughts are with all those caught up in it,” he later Tweeted.

The White House said US President Barack Obama had also been briefed about the Sydney situation from his top counter-terrorism adviser, Lisa Monaco, who had been speaking with counterparts in Australia.

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Mr Obama’s pressing concern was the safety of his citizens from the usually well guarded US consulate in Martin Place. The consulate issued an emergency warning to its citizens “to maintain a high level of vigilance and take appropriate steps to enhance personal security”.

The prime minister of Canada Stephen Harper, whose country suffered a deadly lone wolf jihadist attack in October, also used social media to offer support.

“Canada’s thoughts and prayers are with our Australian friends,” he said via Twitter.

India’s Prime Minister Narendra Modi, whose police yesterday just arrested ISIS’s alleged global social media publicist, also tweeted the events in Sydney were disturbing.

“Such acts are inhuman and deeply unfortunate. I pray for everyone’s safety,” he wrote.

New Zealand Prime Minister John Key was one of the first world leaders to contact Mr Abbott.

The website of British tabloid The Mirror was the first to pick up the story overseas, giving live updates of the “Sydney cafe siege”.

The Mirror describes the area as being “swarmed with police and live television footage showed cafe customers holding a black flag against the window”.

The BBC website is also leading with the siege, reporting that “a gunman has taken several people hostage at a chocolate shop in the Australian city of Sydney”.

“Pictures on Australian television show at least three people with their hands up against a window, and a black flag with Arabic writing,” the BBC reports.

London’s Sun was also running the story, reporting that “terrified hostages were forced to hold an Islamic flag up against the windows of a cafe during an armed siege in Sydney”.

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The New York Times tweeted their report on the siege, which described how armed police officers had “surrounded a cafe in Sydney’s central business district after one or more gunmen took hostages and displayed a black flag with Arabic script in white in the cafe window.

US broadcaster CNN reported that a “major police operation was under way in Sydney”.

“CNN affiliate Seven Network reported that officers were responding to a hostage situation,” the website said.