tweets, external: The Opera House has decided to cancel the children's performances scheduled for Tuesday 16 December, 2014. More: http://on.fb.me/1ISqQAw

A black flag was the battle flag of the Prophet Muhammad and was carried into battle by many of his companions. Today a black flag with the white Arabic text of the "shahada", the basic statement of the Islamic faith, is used as a symbol of engagement in jihad, in the sense of holy war, by militant groups including al-Qaeda and Islamic State. The flag raised in the window of the coffee shop is similar but different to that of IS, which bears the first part of the shahada and the seal of the Prophet below it.

Image source, AP

Frank Gardner
BBC security correspondent

tweets, external: Only last year Sydney hosted International Hostage Negotiators conference. Many countries have rehearsed this situation. #SydneyCafeSiege

The BBC News website's Australia editor Wendy Frew describes "uneasy crowds" at the scene of the hostage taking earlier on Monday. The atmosphere in Martin Place itself was surreal, she says. Office workers who had been evacuated from their buildings, construction workers from nearby building sites and tourists packed the pedestrian plaza one block away from the Lindt coffee shop.

tweets: , externalSydney CBD is very quiet right now. Many pubs have the hostage situation on TVs. Rowdy bystanders have left Martin Place #sydneysiege

Kathryn Cheem who works part-time at the Lindt cafe, described what it was like seeing her colleagues held hostage. "It shakes me to the bone, and they're people who I hold like another family," she told reporters. "You know I see them every day and hold them close to me.... It's good that I'm not there, but I just kind of wish that I could be there for them and tell them that everything's going to be all right."

People who work in Martin Place or nearby have been asked to work from home on Tuesday because of the hostage incident.

An opinion piece in the Herald Sun, external attacks what it calls the "despicable rubberneckers who took selfies and socked back beers and wine while they gawked at the Martin Place siege 100 metres away."

"Lingering just so you can tweet a pic of yourself to your mates, or worse to grab a few coldies and settle in to watch the utter horror unfold like it's a movie at the cinema, is just deplorable," it adds.

There's a rundown on the BBC News website of what eyewitnesses have told the BBC they have seen so far.

Ben Lopez, an expert in hostage negotiation, tells the BBC News Channel: "One of the reasons the police commissioner doesn't want to speculate too much is because it is very clear that the hostage taker is interested in media coverage. And one of the major things we want to do as hostage negotiators is control the flow of information in and out of the siege location. So if other people are calling in, or if the media is complicating things, it could make it difficult for the negotiators to do their job and to try to reach a safe outcome."

In the news conference, the police commissioner stressed the need for calm. "Clearly reprisal attacks are something that should not happen. At this stage, we need to let everyone just settle down."

Image source, AP

Armed police officers stand guard near the cafe as the hostage situation continues into the night.

You can read the whole statement from the Australian Muslim community condemning the attack on Facebook., external It says: "The is a time for all Australians to stand together and support each other."

tweets:, external Police keep talking about this maybe "going into tomorrow". Anyone have long memories about long(est) hostage situations in Australia?

In Australia the hashtag #illridewith you is trending, as non-Muslims offer to travel with Muslims on public transport if they feel unsafe following the events in Sydney. This is some of the reaction to the hashtag:

@Atozai in Sydney tweets, external: This is my Australia. Being a Muslim this hashtag is the best thing happened today. I Love You Australia. #illridewithyou

@aaniqab in Queensland tweets, external: The #illridewithyou tag is seriously such a lovely approach in such horrid times. I feel safe. #sydneysiege

@rizkysaidRtweets, external: hats off to you guys, you're awesome, faith in humanity restored #sydneysiege #illridewithyou

@Daniel_Hatch in London tweets, external: The #illridewithyou movement absolutely makes me proud to be Australian. The correct social response to a terrible isolated incident.

More from the police commissioner: "I understand that no one has been injured, and if that is true we are grateful."

Police Commissioner Scipione says: "If people are being contacted by hostages, we would ask that they ensure that the gunman inside speaks to our trained negotiators. They are the people that can help in this situation." He tells the news conference he cannot confirm how many people are being held inside, or what their identities are.

Police Commissioner Andrew Scipione tells a news conference: "Our only goal is to get those people who are currently caught in that building out of there safely. Nothing will change."

Mike Baird, Premier of New South Wales, says: "I'm proud of how we have responded as a city, a state and a nation. But my thoughts remain with those caught up in this event."

ABC News in Australia is now reporting that the suspect is known to NSW Police. It says one of the country's commercial stations has received video from inside the cafe but are not broadcasting it.