The assassination attempt on Gabrielle Giffords was the latest in a series of mass shootings that have rocked the US.
The man accused of trying to assassinate a congresswoman in an Arizona shooting spree that left six dead appeared in court Monday but said nothing to shed light on his motive.
President Barack Obama led the Americans in a somber minute of silence to honor the 20 people gunned down in Tucson -- where he will attend a memorial service on Wednesday, the White House announced.
Flags were at half-staff at the Capitol in Washington, where hundreds of aides and lawmakers crammed the storied steps of Congress to pay silent tribute to the victims, including one of their own.
Democratic lawmaker Gabrielle Giffords, 40, was shot through the head at point-blank range before the gunman sprayed a crowd of constituents with bullets, a nine-year-old girl and a federal judge among six who died.
Jared Loughner, who faces the death penalty, appeared, his head shaven, amid tight security around the federal court in the Arizona state capital Phoenix.
Wearing a brown prison jumpsuit for the less than 15 minute hearing, 22-year-old Loughner appeared to follow proceedings closely, but said little beyond answering "Yes" to basic questions from judge Michael Anderson.
The judge agreed to his request to have Judy Clarke, who represented the Unabomber -- an anarchist serving life without parole for a 20-year mail bombing spree -- and 9/11 conspirator Zacarias Moussaoui, as his attorney.
No plea of guilty or not guilty was entered during the hearing and a preliminary court appearance was scheduled for January 24. A mug shot released by police showed the gunman with a haunting smile.
Doctors meanwhile said Giffords was still responding to basic commands such as squeezing medics' fingers, fueling growing hope for her recovery, though she remains in a critical condition.
Authorities said Loughner, a troubled young man booted out of a community college last year, fired a full clip of 31 shots.
He was reloading another clip into his 9mm Glock semi-automatic pistol when bystanders, including a 74-year-old retired colonel whose head had just been grazed by a bullet, brought him to the ground.
Obama praised the "extraordinary courage" of those who tackled the gunman, including a young Giffords aide and a woman who helped disarm him.
Outside the Tucson hospital where Giffords clung to life with part of her skull removed, tearful well-wishers gathered, praying and hoping.
A search of Loughner's home on Saturday unearthed a trove of evidence in a safe, according to the criminal complaint, including a letter from Giffords thanking him for attending one of her earlier public meetings, in August 2007.
Investigators also found an envelope with the hand-written notes, "I planned ahead," "My assassination" and "Giffords," the affidavit said.
Officials declined to assess Loughner's motives or mental state. He wrote a stream of barely coherent postings on the Internet that showed an interest in developing a new currency and criticism of "illiterate" fellow residents.
Those who knew him at Pima Community College last spring described a pattern of strange behavior that led many to wonder if he was on drugs.
"If you've ever seen someone hallucinate, it was kind of like that," said former classmate Lydian Ali, 26, adding that he never felt threatened by Loughner but that a woman had dropped out of their poetry class because of him.
"He had this kind of hysterical laugh. ... It was very creepy, very bizarre," said Pima math professor Ben McGahee, who taught Loughner algebra last summer.
A US senator meanwhile said he plans to present legislation to ban high-capacity ammunition clips of the type used by the Tucson gunman.
"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," said Democratic Senator Frank Lautenberg.
Giffords, who narrowly won re-election last year over a favorite 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 made huge gains in November midterm elections, denounced the attack and suspended proceedings of the House of Representatives, whose new leadership took over just three days earlier.