This is how the Sydney siege unfolded. All times are in Sydney time.
9.45am Sydney Time, 11.45am New Zealand Time A number of hostages held inside a cafe in Sydney's Martin Place with an Islamic flag pressed up against a window.
Hostages seen with their arms in the air inside the Lindt Chocolat Cafe and the CBD goes into lockdown.
Hostages pressed against the window in the Sydney Lindt cafe. Photo / Channel 7
Workers in nearby buildings ordered to evacuate the area.
Police establish a 150m exclusion zone around the cafe with specialist police outside the shop.
Martin Place train station shut down.
10.45am Sydney Opera House evacuated following reports of a suspicious package.
10.47am Transport NSW advises motorists and public transport commuters to stay away from the CBD.
10.55am The flag being displayed appears to be the Shahada flag which has been adopted by extremist groups such as the al-Qaeda-linked al-Nusra Front.
11.00am Reports of up to 13 hostages inside the cafe.
11.15am US Consulate sends emergency message to US citizens in Australia.
11.20am Flights diverting around Sydney CBD but operating on schedule.
11.25 The National Security Committee of Cabinet convenes for briefings.
11.30am Prime Minister Tony Abbott issues a statement saying it is a "deeply concerning incident" but reassures the public that security agencies are well trained to handle it.
11.35am Lindt Australia CEO Steve Loane estimates there are up to 50 people being held.
11.35am A number of Australian Muslim leaders offer to help in any way they can.
11.40am Channel Seven air footage of alleged armed offender. He is middle aged with a salt and pepper beard, wearing a headband with Islamic writing.
11.51am NSW Police try to make contact with the people inside the cafe. They advise workers who are trapped in buildings inside the police cordon to stay away from windows.
12.00pm Macquarie Radio reports an alleged gunman has told negotiators he has "devices all over the city" and wants to "speak with the prime minister live on radio".
Several sites around the CBD evacuated, including Channel Seven building, Opera House, NSW parliamentary executive offices, State Library and NSW Supreme Court's criminal courts.
12.02pm All public schools asked to assume "white level" lockout, which means no school group is to leave the school grounds.
A man is arrested 200m from the siege with what appears to be a handgun. The arrest is not linked to the siege.
12.55pm Prime Minister Tony Abbott says there are 'some indications' it is politically motivated but urges Australians to go about their business as usual.
Opposition Leader Bill Shorten offers Tony Abbott the Opposition's full support in dealing with the incident
12.57pm US President Barack Obama is briefed on the siege by Homeland Security and Counter-terrorism chief Lisa Monaco.
1.05pm Canadian Prime Minister Stephen Harper says his country's "thoughts and prayers are with our Australian friends".
1.30pm NSW Police Commissioner Andrew Scipione says they had moved to a "footing" similar to dealing with a terrorist attack. - New Zealand Prime Minister John Key sends a message of support.
1.32pm Lindt stores elsewhere around the country shut for the day.
1.50pm Queensland's police commissioner Ian Stewart says he has information an improvised explosive device may be involved.
1.55pm NSW Premier Mike Baird says Sydney is being tested today, but 'we will remain a democratic society'
2.17pm The hostage drama causes chaos across the CBD, with roads closed and public transport services disrupted.
Indian Prime Minister Narendra Modi labels the events as "inhuman & deeply unfortunate. I pray for everyone's safety"
2.30pm Major banks including CBA, Westpac, NBA and ANZ close several CBD branches but the Reserve Bank of Australia, near the Lindt cafe, is operating with back-up procedures in place. Several retailers around Martin Place are also forced to close.
2.45pm Lindt Chocolat Cafe Australia thank the public for their support via Facebook, saying they are "deeply concerned over this serious incident"
2.57pm The Grand Mufti of Australia puts out a statement condemning the incident, saying "such actions are denounced in part and in whole by Islam."
3.14pm Opposition Leader Bill Shorten calls for the nation to be united, saying Australians are shocked, but they won't be shaken.
3.37pm Three male hostages run out of the cafe.
A hostage holds up her hand to signal the police force not to shot after running out of the cafe Lindt in Sydney. Photo / AP
3.50pm NSW Police Deputy Commissioner Catherine Burn confirms negotiators have spoken with a gunman.
She says there are fewer than 30 hostages and none have been harmed at this stage.
Police refuse to label the attacks as "terrorism" or speculate on how many offenders are inside the cafe.
Police ask people in buildings facing Martin Place to remain there, while everyone else can go home as the working day ends.
4.17pm The National Security Committee of Cabinet meets. AFP chief Andrew Colvin and Justice Minister Michael Keenan are en route back from PNG to Australia
5.00pm Two female Lindt workers leave the cafe