Police in Australia say at least one man is holding several people hostage in a cafe in Sydney's Central Business District, prompting a huge security response in an area home to many government offices.
Police say contact has been made with the hostage-taker, who a witness says is armed, as hundreds of armed police swarm Martin Place, close to the New South Wales state parliament and the neighbourhood where the Reserve Bank of Australia is located.
So far five people - three patrons and two staff members - have emerged from the cafe building, hours after television footage began showing people inside the Lindt Cafe standing with their hands pressed against the windows. The window was later cleared of people.
Tony Abbott, Australian prime minister, called a meeting of the country's national security committee in response to Monday's cafe siege and called the events "deeply concerning" in a statement.
Al Jazeera's Andrew Thomas, reporting from Sydney, said what appeared to be a black flag or sheet with Arabic writing was held up against the cafe's window.
Some observers identified the flag as the al-Raya, which is a generic symbol of Islam that bears the Shahada, a statement of the Islamic faith that says: "There is no god but God and Muhammad is the messenger of God."
"Australia is a peaceful, open and generous society," Abbott said in an address screened on television.
"Nothing should ever change that. I would urge all Australians today to go about their business as usual."
Abbott said it was unclear whether the incident was politically motivated but there were "some indications it may be."
Our correspondent said Abbott had been careful to avoid speculation and not talk about operational details.
"A political motivation could be anything, frankly, from a terrorist incident to something not at all to do with what's going on in the Middle East at the moment or ISIL or any of these things," he said, referring to the Islamic State of Iraq and the Levant group.
Alleged plots and police raids
Andrew Scipione, New South Wales Police Commissioner, said police were still trying to determine the motivation behind the hostage-taking and that they did not yet know where the hostage-taker was from.
He did not put a figure on the number of hostages being held at the cafe. Several local media outlets had earlier reported that at least 13 people were on the premises.
The police have not yet communicated with the hostage-taker and no demands have been made.
The city's iconic Sydney Opera House was evacuated after the siege began along with several nearby buildings, Al Jazeera's Thomas said, adding that it was likely this was a security precaution.
New South Wales Police confirmed they were responding to an incident, announcing on Twitter: "A police operation is under way in Martin Place, Sydney's CBD. People are advised to avoid the area."
Footage from local broadcasters showed scores of people streaming out of the area.
Australia is on high alert for attacks on its soil due to its support for the US-led campaign against ISIL, and police have recently launched a series of high-profile raids in major cities.
"One of the intercepts, heavily reported by Australian media leading to the raids, was an alleged plot to carry out a beheading in Martin Place," our correspondent said.