A mother of a hostage at Sydney's Lindt Cafe received in text from her son who was trapped inside on Monday.

Several people were taken hostage at the caf in the heart of Sydney in south-east Australia at least by one gunman, according to media reports.

"Mum, I am inside the Lindt cafe in Sydney,' an apprentice plumber sent a text message to his mother, identified as Mel, The Daily Telegraph reports.

"My heart stopped, I sent back a message: 'What is going on? Are you okay'."

"I'm okay mum, can't talk," replied the son around 3 pm (Australia time), according to the report.

The woman said that she had not heard from her son after that. Witnesses claimed hearing loud bangs that sounded like gun shots from the cafe. However, the police has said that the hostages were not harmed.

They did not give a confirmation on the number of those taken hostage. Three hostages have managed to escape.