US congresswoman Gabrielle Giffords was shot in the head and a number of other people were injured today when an assailant opened fire as the Democrat was meeting with constituents in Arizona, congressional officials said.

The officials said one of the victims died soon after the attack and others were taken to a nearby hospital. A hospital spokeswoman said Ms Giffords was alive and in a critical condition after the shooting.

The officials said the wounded included some of Ms Giffords's aides who were with her at the time.

One official added the attack was carried out with an automatic weapon.

The FBI and local law enforcement were investigating the attack, which took place while Ms Giffords was greeting constituents outside a shop in Tucson.

Ms Giffords, 40, was re-elected to her third term last November. She was a member of the Arizona House and Senate before coming to Washington.

She first won election to Congress amid a wave of Democratic victories in 2006. In November, she edged a tea party favourite.

Commenting on the attack, newly elected House Speaker John Boehner said: "I am horrified by the senseless attack on Congresswoman Gabrielle Giffords and members of her staff.

"An attack on one who serves is an attack on all who serve. Acts and threats of violence against public officials have no place in our society. Our prayers are with Congresswoman Giffords, her staff, all who were injured, and their families. This is a sad day for our country."

The shooting came amid a highly charged political environment that has seen several dangerous threats against politicians but nothing that reached the point of actual violence.

A San Francisco man upset with House Speaker Nancy Pelosi's support of healthcare reform pleaded guilty to threatening the Democratic congresswoman and her family, calling her directly on March 25 and threatening to destroy her northern California home if she voted for the reform.

In July, a California man known for his anger over left-leaning politics engaged in a shoot-out with highway patrol officers after planning an attack on the American Civil Liberties Union (ACLU) and another non-profit group.

The man said he wanted to "start a revolution" by killing people at the ACLU and the Tides Foundation.

Ms Giffords, a moderate Democrat, has herself drawn the ire of the right, especially for her support of the healthcare Bill.

Her Tucson office was vandalised a few hours after the House vote to approve the healthcare law in March, with someone either kicking or shooting out a glass door and window.

CJ Karamargin, a spokesman for Ms Giffords, said the congresswoman was in surgery.

Congressional officials confirmed that an aide to Ms Giffords was killed in the shooting.

President Barack Obama called the shooting "an unspeakable tragedy" and that such "a senseless and terrible act of violence has no place in a free society".

US Capitol police say the gunman is in custody. He has been identified as Jared Laughner.

Three hours after the shooting, the L-shaped shopping centre in Tucson was blocked off by police and had fire engines and other vehicles in its car park blocking the view of the store's front door.

No shell casings could be seen from the area 500 yards from the store where reporters and photographers were kept.

Shortly before the shooting, Ms Giffords tweeted: "My 1st Congress on Your Corner starts now. Please stop by to let me know what is on your mind or tweet me later."

Ms Giffords is married to astronaut Mark E Kelly, who has piloted space shuttles Endeavour and Discovery. The two met in China in 2003 while they were serving on a committee there, and were married in January 2007.

A girl aged around nine was killed in the shooting, a surgeon has said.

A doctor at the hospital in Tucson said there were five others in a critical condition.

In addition, Ms Giffords was said to be "following commands" but was under anaesthetic following surgery.

He said the congresswoman had been shot "through and through", with the bullet passing through her brain.

"Gabrielle Giffords's condition was very optimistic because she was following commands.

"She was shot one time in the head, through and through. It went through her brain.

"I'm about as optimistic as I can get in this situation. Surgery has been completed. She is under anaesthesia."

A US marshal, David Gonzales, said district Judge John Roll also died in the attack.