The 22-year-old Arizona man accused of shooting 20 people while trying to assassinate a US congresswoman in Tucson on Saturday has appeared in court.

Jared Lee Loughner was charged on five counts, including killing federal employees and attempting to assassinate Democratic Congresswoman Gabrielle Giffords.

He was remanded in custody to appear in court again on 24 January.

No plea of guilty or not guilty was entered during the hearing.

Ms Giffords, 40, is in an induced coma at a hospital in Tucson after suffering a gunshot wound to her head.

Authorities said Jared Loughner fired 31 shots as Ms Giffords met with constituents at a grocery shop on Saturday. He was overpowered as he tried to reload his 9mm Glock semi-automatic pistol, which he had bought at a local store.

Prosecutors said Mr Loughner has been charged with five counts, including murder and attempted murder.

'As the investigation goes on, there may well be additional charges that will be filed,' Robert Mueller, director of the Federal Bureau of Investigation, told reporters in Tucson.

Mr Mueller said that no 'specific threat' remains, although authorities have urged politicians to be careful. Police have also questioned a taxi driver seen entering the Safeway store with Loughner but later concluded he was only seeking change for the fare, sheriff's deputy Jason Ogan said.

Prosecutors said Jared Loughner went to a similar public meeting with Ms Giffords in August 2007. Investigators searched a safe at Loughner's home, where he lived with his family, and found a letter from Ms Giffords thanking him for his attendance.

Also in the safe they found an envelope with the handwritten notes, 'I planned ahead,' 'My assassination' and 'Giffords,' it said.

US President Barack Obama today led US citizens in observing a moment of silence for victims of the attack in Arizona.

At 11am (4pm Irish time), Mr Obama and his wife Michelle emerged from the White House, took a few steps toward the South Lawn and stood, heads bowed, the only sound the somber ringing of a bell.

After the observance, they returned to the building without making a statement.

Political climate criticised

Pima County Sheriff Clarence Dupnik has criticised the political climate in Arizona, which shot to the international spotlight last year over tough laws cracking down on illegal immigration from Mexico, an hour south of Tucson.

'The rhetoric about hatred, about mistrust of government, about paranoia of how government operates ...has an impact on people, especially who are unbalanced personalities to begin with,' said Mr Dupnik, a member of President Obama's Democratic Party.

Ms Giffords, who narrowly won re-election last year over a favourite of the conservative Tea Party movement, is a centrist Democrat who supports increased border security and, incidentally, loose restrictions on gun ownership.

Lawmakers of the rival Republican Party, which swept last year's election, denounced the attack and suspended proceedings of the House of Representatives whose new leadership had taken over just three days earlier.

John Boehner, the new house speaker, said in his Ohio constituency that an 'attack on one who serves is an attack on all who serve.'

'No act, no matter how heinous, must be allowed to stop us from our duty,' he said.

Doctors said it is too early to say how long Ms Giffords' recovery might take. Many patients with such serious wounds need months to return to anything close to normal life.

But the doctors have voiced optimism after her rapid progress, helped in part by the fact that the bullet did not go through both hemispheres of her brain.

'Because of that, Congresswoman Giffords is able to communicate with us... following simple commands. We're very encouraged by that,' said Michael Lemole, the neurosurgeon who operated on her at University Medical Center.

'Brain swelling at any time can take a turn for the worse. But I am cautiously optimistic,' he told reporters.

Authorities hailed the courage of Patricia Maisch, who was waiting to have her photograph taken with the congresswoman when Loughner opened fire. Ms Maisch grabbed the bottom of the suspect's weapon to prevent him from reloading.

But six people were killed and at least 14 others were injured.

Among the dead was federal judge John Roll, who came to speak with the congresswoman, and a nine-year-old girl.

The girl, Christina Taylor Green, was born on 11 September, 2001.

Her parents said her accident of birth gave her a precocious interest in politics, explaining why she went to Ms Giffords' public event.