TUCSON, Ariz. - Hundreds of people packed a Tucson cathedral on Sunday for a service to remember a bloody morning one year ago when a gunman's deadly rampage shook a community and shocked a nation.

Girls in white dresses and red sashes danced down the aisle as a song called "Hero in the Dark" played, and a pastor called on everyone to celebrate the lives of the people lost and those who acted heroically during the shooting that left six dead and 13 wounded, including U.S. Rep. Gabriele Giffords.

The names of those killed were read as a bell rang for each of them, and their family members and survivors walked down the aisle with red roses and placed them in a vase at the front of the church.

"We remember, we remember, we remember with grateful hearts," those gathered chanted together, standing, many closing their eyes.

"Even in the midst of this troubling year, the healing, the courage that we have experienced in our community -- each one of us can notice how our cups overflow with the blessings of our lives," said Stephanie Aaron, Giffords' rabbi.

Ron Barber, a Giffords staffer who survived two gunshot wounds, said he woke up Sunday dreaming about Giffords, who was severely wounded, and Giffords staffer Gabe Zimmerman, who died.

"You have to think about the six people whose loved ones don't have them today," said Barber at St. Augustine Cathedral just before the service began. In the crowd were survivors, families and others, including Arizona Gov. Jan Brewer.

It's been a year of reflecting on lives shattered, of struggling with flashbacks and nightmares, of replaying the what-ifs before the deadly rampage. Among those killed were a federal judge and 9-year-old Christina-Taylor Green, born on Sept. 11, 2001.

Many throughout the close-knit southern Arizona community began the day of remembrance Sunday by ringing bells at 10:11 a.m. (1711 GMT), the exact time the gunman shot Giffords and methodically mowed down a line of people waiting to talk to her during a congressional meet-and-greet outside a Tucson supermarket on Jan. 8, 2011.

Bruce Ellis and his wife Kelly Hardesty, both 50, wept as the bells rang at the Safeway supermarket where the shooting occurred. They held each other tight.

"It's shocking to have a massacre like this occur in your backyard," Ellis said. "It's something that happens on the news, not in your neighbourhood."

About 30 others rang hand-held bells, hugged each other and cried as the time of the shooting passed. Many bowed their heads in prayer.

Giffords and her husband, former astronaut Mark Kelly, went to the scene of the shooting Saturday, and Kelly tweeted a photo of Giffords pointing, he says remembering where she had parked that day. They went to University Medical Center, where Giffords was treated after the attack, and visited a trailhead outside Tucson named in honour of slain staffer Zimmerman.

The couple will join thousands at an evening candlelight vigil at the University of Arizona. Kelly was expected to speak.

Barber said he spent time with Giffords on Friday and Saturday, mostly reflecting about Zimmerman's life.

"Even though it's a hard weekend for her and all of us, she wanted to be here with her community to remember," he said. "She's sad, we're all sad, and she's glad to be home."

Daniel Hernandez, Giffords' former intern who came to her aid after the shooting and has been hailed as a hero, called Sunday a solemn day of remembrance and an opportunity to allow Tucson and those affected by the shooting to heal further.

"It's definitely been a really difficult time for all of us," he said. "But last time this year, there was a lot of anger. And now it's, 'How can we heal and move forward?"'

President Barack Obama called Giffords on Sunday to offer his support and tell her he and Michelle Obama are keeping her, the families of those killed and the whole Tucson community in their thoughts and prayers, according to the White House. He called Giffords an inspiration to his family and Americans across the country.

The 41-year-old Giffords has spent the last year in Houston undergoing intensive physical and speech therapy. Doctors and family have called her recovery miraculous; she is able to walk and talk, vote in Congress and gave a televised interview to ABC's Diane Sawyer in May. But doctors have said it would take many months to determine the lasting effects of her brain injury. The three-term congresswoman has four months to decide whether to seek re-election.

"She's making a lot of progress. She's doing great," said Rep. Debbie Wasserman Shultz, a close friend. "She still has a long way to go."

Jared Lee Loughner has pleaded not guilty to 49 charges in the shooting. The 23-year-old, who has been diagnosed with bipolar disorder, is being forcibly medicated at a federal prison facility in Missouri in an effort to make him mentally ready for trial.

Many of the survivors have lobbied for gun legislation in Washington in hopes of preventing similar shootings and started various nonprofits that award scholarships, help needy children and promote awareness about mental illness.

Sunday's events were designed to bring Tucson residents together much like they came together after the shooting last year.

Albert Pesqueira, assistant fire chief for the Northwest Fire District in Tucson, was one of the first responders to the shooting. He came to the Safeway on Sunday to remember and to heal.

His most vivid memories from that day are the sounds of moaning and crying among shooting victims in the aftermath of the attack.

"I can still hear them," Pesqueira said. "We'll never be the same. We'll never be normal again because of what occurred."