A Calgary man may lose his arm after being mauled by a tiger.

Police say two men broke into the Calgary Zoo grounds just after one-am Monday morning.

They scaled a series of fences and managed to spook a two year old male Siberian Tiger.

Two year old Vitali is one of three Siberian tigers at the Calgary Zoo.

He was awoken by a pair of unexpected visitors. The two 27 year old men snuck into the zoo, scaled a 15 foot perimeter fence lined with barbed wire and then they hopped a second safety fence to get right up against the tiger cage.

Dr. Sandi Black, the Calgary Zoo's veterinarian says "the gentleman was hooked by a claw, and the arm was dragged in and then continued to be attacked from that point".

The men struggled to get loose from the tiger's grip.

And seconds after the attack an on-duty female security guard received a call for help on her personal cell phone. Police say the intruders knew the woman personally but hadn't told her they were at the zoo. The security guard called "911".

One man is now in hospital with serious injuries. The zoo calls the incident unusual and many are still wondering what the two men were thinking.

Jim MacDonald, a visitor at the zoo told CTV News "he had it coming, fooling around with stuff he shouldn't have been fooling around with".

The incident has Zoocheck Canada calling for better safety around animal enclosures.

The Calgary Zoo is reviewing its procedures and safety measures.