Snap happy Sydneysiders have been slammed on social media for posting what appear to be "siege selfies" taken in the area of the Lindt café hostage crisis in Martin Place.

Since before 10am this morning, a number of people, believed to be up to 30, have been held against their will by an armed man in the Sydney CBD cafe. Five hostages have since escaped the building but several more are still inside.

A group of friends took this selfie while standing behind police tape. (Twitter) (Twitter)

In one of a number of photos to surface on Twitter this afternoon, three mates are seen smiling into the camera with police tape in full view behind them.

Two friends take a selfie near Martin Place siege. (Twitter) (Twitter)

In another photo, two women are seen posing with Martin Place behind them. While a third shows a man happily snapping the scene amid a sea of onlookers.

A man is shown taking a selfie near the scene. (Twitter)

The photos have been heavily criticised by social media users.

"Don't go home and hug your loved ones and be grateful that you're not caught up in a hostage nightmare… go to the scene and take a selfie," one Twitter user wrote.

Another wrote: "Awful, do they not have any feelings. People are being held hostage and all they can do is take a selfie."

While another wrote: "Just hangin out at a dangerous hostage crisis, think I'll stop and take a selfie, with a smile of course."

Yet another person wrote: "I love a selfie, but how is this ok?"