Saying he wants to live up to the dreams of a 9-year-old girl who perished in a Tucson parking lot last weekend, President Obama on Wednesday urged Americans to temper the political discourse now polarizing the public square.

In honoring the heroes and victims of a mass shooting that left six dead and 14 injured, including Arizona Rep. Gabrielle Giffords, the president directly addressed the ensuing national debate about rhetorical vitriol by pleading with Americans not to "use this tragedy as one more occasion to turn on one another."

"At a time when our discourse has become so sharply polarized -- at a time when we are far too eager to lay the blame for all that ails the world at the feet of those who think differently than we do -- it's important for us to pause for a moment and make sure that we are talking with each other in a way that heals, not a way that wounds," he said during a 34-minute speech to a capacity crowd at the University of Arizona's sports arena, the McKale Memorial Center.

Nearly 30,000 people gathered, indoors and outdoors, for the nighttime service in honor of the victims in Saturday's mass shooting, which shook the nation. Jared Lee Loughner, 22, is being held without bail on charges he opened fire at a Tucson grocery store where Rep. Gabrielle Giffords was having a town-hall style event.

In a stunning moment, Obama announced that Giffords, who is in critical condition, opened her eyes for the first time after he left her intensive care hospital room before the service. First lady Michelle Obama held hands with Giffords' husband, Mark Kelly, as the news brought soaring cheers throughout the arena.

"She knows we are here and she knows we love her and she knows that we will be rooting for her throughout what will be a difficult journey," he said to thunderous applause and a standing ovation.

Obama bluntly conceded that there is no way to know what triggered the mass shooting. He tried instead to leave indelible memories of the people who were gunned down and to rally the country to use the moment as a reflection on the nation's behavior and compassion.

"I believe we can be better," Obama said. "Those who died here, those who saved lives here -- they help me believe. We may not be able to stop all evil in the world, but I know that how we treat one another is entirely up to us."

Obama cited 9-year-old Christina-Taylor Green, the youngest of the six killed, as a role model for America.

"I want us to live up to her expectations," he said. "I want our democracy to be as good as Christina imagined it. I want America to be as good as she imagined it. All of us -- we should do everything we can to make sure this country lives up to our children's expectations."

Obama noted that Christina was one of 50 babies born on Sept. 11, 2001 who were pictured in a book called "Faces of Hope."

"If there are rain puddles in heaven, Christina is jumping in them today," he said.

One of the first speakers at the service was Daniel Hernandez Jr., an intern for Giffords who has been called a hero for applying pressure to the Arizona Democrat's wound after she was shot in the head. He delivered a brief but spirited speech in which he said he wasn't a hero, giving that label instead to the first responders to the attack and surgeons who saved Giffords' life.

"The one thing we have learned from this great tragedy is we have come together," he said. "On Saturday we all became Tucsonians. On Saturday, we all became Arizonans. And above all, we all became Americans."

Spectators also roared when Pima County Sheriff Clarence Dupnik was shown on the large screen. Dupnik drew praise and criticism after he blamed the rampage in part on extreme political rhetoric, bigotry and hatred.

Before Obama delivered his speech, Homeland Security Secretary Janet Napolitano spoke, quoting a passage from the book of Isaiah. Attorney General Eric Holder also spoke in advance of the president.

Obama spent a total of 45 minutes at University Medical Center, including a nine-minute visit with Giffords and her husband, astronaut Mark Kelly. Obama also met with several other patients along with doctors and nurses at the hospital.

"The president wanted to begin this solemn trip by stopping first at the hospital where Congresswoman Giffords and others continue to recuperate," Gibbs said.