World reacts with shock and disbelief following Martin Place cafe siege

There has been shock around the globe over the Sydney siege, with the world watching on as one of the Australia's darkest days unfolded.

From the United States to Britain - there was disbelief that this could happen in a country renowned for its peace and freedom.

News organisations around the world lead with story, beaming live vision from Martin Place into the homes of millions.

When the news broke the New York Police Department moved quickly, deploying more officers to high profile locations like the Empire State Building and Times Square.

"(We've deployed) a lot of intelligence-driven, strategically based high profile police coverage," NYPD deputy commissioner John Miller said.

While US President Barack Obama had this message for ISIL.

"Make no mistake, our coalition isn't just going to degrade this barbaric terrorist organisation, we're going to destroy it," Mr Obama said.

Other world leaders expressed their concern via social media, with British Prime Minister David Cameron describing the situation as "deeply concerning".