US President Barack Obama mourned victims of the Arizona shooting last night and urged the US not to let a political debate over the tragedy be used as 'one more occasion to turn on one another'.

The president made an emotional address to thousands of people who attended a memorial service in Tucson.

Mr Obama said no-one knew what prompted a gunman to go on a rampage that killed six people and critically wounded congresswoman Gabrielle Giffords.

He warned against seeking 'simple explanations' and cautioned US citizens not to place blame.

'None of us can know with any certainty what might have stopped those shots from being fired, or what thoughts lurked in the inner recesses of a violent man's mind,' Mr Obama said.

Jared Lee Loughner, 22, has been charged with firing at Ms Giffords and others gathered in a Tucson shopping centre on Saturday where the 40-year-old Democrat was hosting a meet-and-greet for constituents.

Among those killed were a young girl, a federal judge and an aide of Ms Giffords.

Mr Obama paid tribute to nine-year-old Christina Taylor Green, who was born on 11 September, 2001, and had attended the event because she was interested in government.

'I want us to live up to her expectations,' Mr Obama said. 'I want our democracy to be as good as she imagined it.'

Mr Obama began his visit to Arizona by visiting Ms Giffords, who survived a gunshot to the head that travelled the length of her brain on the left side.

He also visited four other patients wounded in the attack.

Mr Obama brought roars of approval from the estimated crowd of 14,000 people inside a University of Arizona arena by saying he had been told that Ms Giffords opened her eyes for the first time since the shooting.

'Gabby opened her eyes,' he said. 'So I can tell you she knows we are here, she knows we love her.

'She knows that we are rooting for her through what is undoubtedly going to be a difficult journey.'