Hostages held in siege at cafe in Sydney's Martin Place

A number of hostages have been taken in a siege underway in Sydney's CBD.

At least three people have been seen through the windows of a cafe in Martin Place with their hands raised.

Prime Minister Tony Abbott has spoken for the first time about the siege in central Sydney.

"We don't yet know the motivation of the perpetrator, we don't know whether this is politically motivated although obviously there are indications it could be," he said.

Key points:

•Heavily armed police have taken up position around the Lindt cafe in central Sydney

•Hostages have been seen inside the cafe with their hands in the air

•Black flag with white Arabic writing reading "There is no God but Allah" and "Mohammed is the messenger of God" waved behind the hostages

•Martin Place and Sydney Opera House have been evacuated

•Lindt Australia executive says there are about 10 staff and up to 30 customers trapped in the cafe

•National Security Committee of Cabinet has convened; Prime Minister Tony Abbott described incident as "deeply concerning" and offered NSW all assistance possible.