World leaders including Indian Prime Minister Narendra Modi and New Zealand Prime Minister John Key have tweeted messages of support to the people caught in today's siege in central Sydney.

It was confirmed that US President Barack Obama had also been briefed about the situation earlier in the day.

The incident in Sydney is disturbing. Such acts are inhuman & deeply unfortunate. I pray for everyone's safety. — Narendra Modi (@narendramodi) December 15, 2014

I’ve contacted Prime Minister @TonyAbbottMHR to let him know our thoughts are with the people of Australia today. — John Key (@johnkeypm) December 15, 2014

Canada's thoughts and prayers are with our Australian friends. #MartinPlace#SydneySiege — Stephen Harper (@pmharper) December 15, 2014

We continue to monitor the situation in #Sydney closely. I've been in touch with @JulieBishopMP and expressed our thoughts and prayers. — John Baird (@HonJohnBaird) December 15, 2014

In Australia, Prime Minister Tony Abbott expressed his concern over the situation at a press conference.

"This is a very disturbing incident," he said.

"I can understand the concerns and anxieties of the Australian people at a time like this, but our thoughts and prayers must above all go out to the individuals who are caught up in this.

"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."

Lindt Chocolate Cafe released a statement a short time ago, thanking people for their support of its staff and customers at Martin Place.

Australian politicians have also used Twitter to express their support:

Terrifying situation in Martin Place. Our thoughts are with the people being held hostage, their families and friends. — Adam Bandt (@AdamBandt) December 15, 2014