A US congresswoman is in critical condition after a gunman shot her in the head and killed six people in an Arizona shooting spree.
Democratic Representative Gabrielle Giffords was shot at close range at a public meeting in a Tucson supermarket.
The 40-year-old is under sedation, but the doctor treating her said he was "very optimistic about her recovery".
A 22-year-old man has been arrested and police are hunting for a second man in connection with the shooting.
The suspect in custody, who was wrestled to the ground at the scene of the shooting, has been named locally as Arizona resident Jared Loughner, who is described as having a troubled past.
Various former classmates have described him as "obviously disturbed" and a loner who had posted a number of anti-government videos and messages on social networking websites.
'Outrageous bigotry'
Local police, who have not confirmed the suspect's name, said they were hunting for a possible accomplice, who may have driven the suspect to the supermarket.
They have released a CCTV image of a dark-haired man, described as aged between 40 and 50 and wearing a blue jacket and jeans at the time of the attack.
Left-wing commentators have accused right-wing politicians of creating a climate of hatred that allows such attacks.
The rhetoric that instantly sprang up around the shootings shows the volatile, febrile state of American politics, says BBC North America editor Mark Mardell in Washington.
Pima County Sheriff Clarence Dupnik said a consuming atmosphere of political vitriol centred on Arizona may have been a factor.
"When you look at unbalanced people, how they respond to the vitriol that comes out of certain mouths about tearing down the government; the anger, the hatred, the bigotry that goes on in this country is getting to be outrageous," he said.
"And unfortunately, Arizona, I think, has become the capital. We have become the Mecca for prejudice and bigotry."
This anger had spilled into violence before, with Ms Giffords' office being vandalised last March after she upset Arizona conservatives by supporting Mr Obama's healthcare reform bill.
Sheriff Dupnik said the congresswoman had been threatened by someone with a gun during her re-election campaign in November, adding that there had been other threats.
House Speaker John Boehner vowed on Sunday that the shootings would "not deter us from our calling to represent our constituents".
He added: "No acts, however heinous, must be allowed to stop us from doing our duty."
Former Alaska Governor Sarah Palin, a conservative Republican, had placed Ms Giffords on a list of politicians she wanted to remove from office in November's mid-term polls.
As a moderate Democrat who narrowly defeated a so-called Tea Party candidate in the elections, Ms Giffords had referred to the hatred during an interview with MSNBC.
"We're on Sarah Palin's targeted list, but the thing is, that the way that she has it depicted has the crosshairs of a gun sight over our district. When people do that, they have to realise that there are consequences to that action," she said.
Mrs Palin has issued a statement offering her condolences to the families of Ms Giffords and the other victims of the attack.
Indiscriminate shooting
The attacker struck as Ms Giffords held one of her regular open-invitation meetings - which she called "Congress on your corner" - with her constituents outside the Safeway supermarket in Tucson at around 1000 (1700 GMT) on Saturday.
Eyewitnesses say the gunman approached Ms Giffords and shot her at close range. Later reports from the hospital said a bullet passed through her head without exploding.
The attacker then fired some 20 shots indiscriminately before being overpowered by members of the crowd as he attempted to reload what one witness described as a semi-automatic Glock pistol.
He was then hauled away by police, witnesses said.
The six dead included a nine-year-old girl, federal Judge John Roll - who had spent more than 40 years on the bench - and one of Ms Giffords' political aides.
In addition to the six deaths, police said 12 people - including Ms Giffords - had been wounded.
Outside the Tucson hospital where the victims were taken, several people have laid candles held an overnight vigil. A few miles away, both the scene of the attack and some streets around the suspect's home - where he lived with his parents - remain cordoned off.
Eyewitness accounts
As the US comes to terms with the tragedy, witnesses have been giving their accounts of the attack.
Alex Villec, a 19-year-old volunteer at the event, said he had been approached by a "shady-looking" man wearing a black cap and baggy trousers who asked to speak to the congresswoman.
He told the man to wait in line, but said he returned moments later and burst through a table separating Ms Giffords from the public.
Another witness said he saw the gunman shoot Ms Giffords in the head before "just spraying gunfire everywhere".
"At that point I ducked behind the concrete post," said the witness, Dr Steven Rayle. "I acted like I had been shot and was dead so I wouldn't be a target."
One of the men who helped wrestle the gunman to the ground said he had two more ammunition clips and a knife in his pockets.
"He was ready for war, he was not playing around," Joe Zamudio told CNN television. "He was blank. He was callous, almost."
Presidential tribute
US President Barack Obama said the shooting was a "tragedy for our entire country", and urged the nation to "come together and support each other".
"I ask all Americans to join me and Michelle in keeping all the victims and their families, including Gabby, in our thoughts and prayers," he said.
He described Ms Giffords as a personal friend and an "extraordinary public servant", and tasked FBI director Robert Mueller with overseeing the investigation.
Ms Giffords, who represents the eighth district of Arizona in the House of Representatives, is married to space shuttle astronaut Mark Kelly.
Described as a "a rising star" in the Democrats with hopes of eventually winning the Arizona Senate seat, she has served on several congressional committees, including those covering the armed services and foreign affairs, and is a member of the "blue dog" group of centrist Democrats.
The House of Representatives majority leader Eric Cantor announced that all of next week's legislative debates would be postponed to allow politicians to "take whatever actions may be necessary in light of today's tragedy".
