The injured victims are described as being in a critical condition

All of the victims were visitors to the zoo, a spokesman for San Francisco emergency services said.

The Siberian tiger, named Tatiana, was the same one that mauled a keeper just before Christmas last year. It has now been shot dead.

The incident occurred as the zoo was closing around 1700 (0100 GMT). It is not clear how the cat escaped its pen.

Emergency services spokesman Lt Ken Smith said that they were called to the zoo at 1715 following a report that a person had been attacked by a tiger.

In response the zoo was evacuated and armed police officers and firefighters sent to the scene.

There were not many visitors to the zoo on Christmas Day, and it was dark by the time police arrived.

A police spokesman said officers found the dead body of one of the victims right outside the tiger's enclosure.

Tatiana attacked one of the zoo's keepers last year

Then they saw the second victim, spokesman Steve Mannina told the Associated Press news agency.

He was about 300 metres away, in front of a cafe, sitting on the ground, with blood running from gashes in his head.

Tatiana sat next to him, and suddenly attacked him again, Mr Mannina said.

Officers approached, and fired at the animal when it began to advance towards them.

The spokesman did not confirm, but it was reported the third victim was found in the cafe.

The three victims were men in their 20s, the spokesman said.

Past attack

The two injured people were described as being in a critical condition.

Margie Shafer, a reporter from local radio station KCBS, said that there were unconfirmed reports that the tiger had jumped a moat to make its escape.

San Francisco Zoo is home to both Siberian and Sumatran tigers.

In December 2006, a keeper at the zoo had her arm severely lacerated when Tatiana reached through the bars of her cage and mauled her during a public feeding.

As a result of that attack the zoo built a new feeding enclosure in its Lion House to protect the trainers as they carried out feedings.