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A man charged with trying to assassinate a US congresswoman in a shooting that left six people dead and more than a dozen wounded has appeared in federal court in Phoenix, Arizona.

Jared Loughner, 22, faces several charges over the attack on Saturday.

Representative Gabrielle Giffords, who was shot in the head, remains in a critical condition but doctors say the swelling in her brain has stabilised.

Among the dead were a nine-year-old girl and a federal judge.

President Barack Obama, who led the US in a silent tribute from the South Lawn of the White House on Monday morning, said a priority for the nation was "making sure we are joining together, pulling together as a country".

The president will travel to Arizona on Wednesday to attend a memorial service for the victims of the Arizona shooting, administration officials said.

Mr Loughner walked into the courtroom wearing handcuffs and in a prison uniform, with a cut on the right side of his head.

During the 13-minute hearing, Mr Loughner said very little, only periodically leaning forward to speak into a microphone.

Mr Loughner, who did not enter a plea, confirmed his identity and had an attorney appointed to defend him.

When asked, he said he understood that he could get life in prison or the death penalty for allegedly killing federal Judge John Roll on Saturday in Tucson, Arizona.

The lawyer assigned to him, Judy Clarke, previously defended Oklahoma City bombing conspirator Timothy McVeigh. She waived a detention hearing for her client.

The federal judge ordered Mr Loughner held without bail and scheduled a preliminary court appearance for 24 January.

The courtroom was under heavy protection on Monday by about a dozen US marshals.

Mr Loughner was charged with five crimes - the attempted assassination of a member of Congress, the first degree murder of two federal employees and the attempted murder of two federal employees.

It is unclear whether the US justice department will seek the death penalty against Mr Loughner.

Mr Loughner registered as an independent voter in Arizona in the autumn of 2006 and voted in US elections in 2006 and 2008, according to the Washington Post newspaper, although some other reports suggested he had registered as a Republican.

Ms Giffords had been holding an open-invitation meeting with constituents outside a supermarket in Tucson on Saturday when a man holding a gun approached and opened fire.

Ms Giffords, 40, was shot from close range by the gunman, who then began shooting into the crowd.

Doctors at the hospital where Ms Giffords is being treated have said they are optimistic about her recovery.

Christina Taylor Green, the young girl killed in the shooting, was born on 9/11 and featured in a book - Faces of Hope, Babies Born on 9/11 - about some of the children born on that day.

A total of 19 people were shot outside the supermarket in Tucson.

Flags across the US were flown at half mast on Monday, and Mr Obama said the nation was "grieving and shocked".

The president praised the "extraordinary courage" of the people at the scene who wrestled the gunman to the ground, saying they had shown "the best of America".

Lawmakers paid tribute to Ms Giffords and other victims of the mass shooting on the steps of the Capitol building.

In Tucson, small groups gathered in public spaces, in offices and in shops and stopped in silence for a minute.

The crew on board the International Space Station (ISS) also paused for a brief silence. The commander on the ISS is Ms Giffords' brother-in-law, Cmdr Scott Kelly.

"As I look out the window, I see a very beautiful planet that seems very inviting and peaceful. Unfortunately, it is not," Cmdr Kelly said.

"We are better than this. We must do better," he added.

Meanwhile, Democratic Senator Frank Lautenberg, a gun control advocate from the state of New Jersey, announced plans on Monday to introduce legislation that would ban high-capacity ammunition clips, like the one used in Saturday's attack.

"The only reason to have 33 bullets loaded in a handgun is to kill a lot of people very quickly. These high-capacity clips simply should not be on the market," Mr Lautenberg said.