A Democrat US congresswoman was last night in surgery in a critical condition while six other people were shot dead by a lone gunman in front of hundreds of people at a public event in Arizona yesterday.

Gabrielle Giffords, (40) a Democratic member of the US House of Representatives, and up to 12 other people, were shot at close range at the event in Tuscon.

American television networks reported last night that an unidentified gunman ran up and began shooting indiscriminately as the congresswoman addressed supporters. It is understood that Giffords was rushed by ambulance to University Medical Center in Tuscon. News reports last night suggested that at least six people had been killed in the shootings, including several members of Giffords' staff.

Giffords, who was first elected in 2006, was hosting a "Congress on Your Corner" event at a supermarket.

Twelve people were injured in the shooting shortly after 10am local time, according to Jason Ogan, a spokesman for the Pima Country Sherrif's department.

The police were sweeping the area for any other possible suspects last night and said that the gunman was being held in custody.

Among those shot were Giffords' district director and local press secretary. According to CNN news, witnesses reported seeing bodies at the scene.

Giffords was taken to University Medical Center in Tucson but her condition remained unclear last night.

The outspoken congresswoman was a target of violence during the health care vote when she had a brick thrown through her window.

Just hours before the shooting, Giffords tweeted about the event.

"My 1st Congress on Your Corner starts now," she wrote. "Please stop by to let me know what is on your mind or tweet me later."

Giffords has been a rising star among Blue Dog Democrats. She recently won a third term in her Republican-leaning district with the benefit of her centrist policies, an engaging personality, and divisions within the local GOP camp.

She has served on the Armed Services Committee. She also chaired the Space and Aeronautics Subcommittee, where she was an active supporter of NASA and space exploration.

Her husband, Mark Kelly, is an astronaut who has flown on the space shuttle.

Giffords was first sworn in as a congresswoman in 2007. She was the third woman in Arizona's history to be elected to serve in the US Congress, and is also an advocate of gun ownership.