Flags were at half-mast at the White House and the Capitol in honour of the victims of the Arizona shooting [EPA]

The US president has led the nation in a moment of silence in honour of the victims of a weekend shooting in Arizona that left six dead and a congresswoman seriously wounded.

Flags were at half-mast at the White House and the Capitol, where the steps of the US congress were crammed with politicians paying tribute to the victims.

Obama and his wife Michelle emerged from the White House at 11am local time (1600 GMT), took a few steps onto the South Lawn and stood, heads bowed, as a bell was rung.

At the Capitol, Emanuel Cleaver, a Democratic congressman, recited a prayer: "We ask blessings on the spirit of this nation ... help us move from this dark place to a place of sunshine ... we ask that you help keep our hearts pure."

Giffords, 40, who represents Arizona's eighth congressional district in the House of Representatives, is said to be doing well in hospital after being shot in the head during the attack in Tucson.

Doctors said on Monday that she was still responding to simple commands and her condition is stable.

Her brain remains swollen, but the pressure has not increased.

Six people, including a US federal judge and a nine-year-old girl, were killed in Saturday's attack, with Giffords and 13 others wounded.

Eight of those injured remain hospitalised.

Jared Loughner, the accused attacker, is to appear in a federal court in the state capital Phoenix later on Monday charged with five counts, including murder and attempted murder.

Investigators said on Sunday they had carried out a search at the suspect's home and seized an envelope from a safe with messages such as "I planned ahead", "My assassination" and the name "Giffords" next to what appears to be the man's signature.

'Dark place'

Following an acrimonious campaign in the run-up to last November's midterm congressional elections, some commentators were quick to cite a shrill climate of political vitriol might have played a role in the shooting.

"We are in a dark place in this country right now and the atmospheric condition is toxic," Cleaver told NBC's Meet the Press.

The suspected attacker opened fire with a semi-automatic pistol at point-blank range outside a supermarket.

Police seeking a motive for the shooting spree were looking at a trail of anti-government messages on the internet left either by Loughner or someone writing under that name.

There was no coherent theme to the messages.

In Tucson, Clarence Dupnik, the Pima County sheriff, said the suspect had made threats to kill in the past but not against Giffords.

Obama has put Robert Mueller, the FBI director, in charge of the investigation.