Witnesses say the suspect, Jared Loughner, started shooting as people lined up to meet Giffords outside a Safeway grocery store.
One of the people in that line was retired army colonel Bill Badger. Badger said he heard the shots and dropped to the ground.
Then he felt a sting to the back of the head as a bullet grazed him.
Unaware that he had been hit, he said he stood up and realized the suspect was standing right next to him.
That's when another Good Samaritan took a folding chair and slammed it into the back of the suspect's head.
Badger said he then grabbed Loughner's wrist to keep him from firing again and with the help of that other stranger, he restrained Loughner until police arrived.
"I had this guy by the throat and the other guy on the other side had his knee right on the back of his neck," said Badger, adding that he had Loughner in a choke-hold.
Badger was reluctant to take too much credit, saying anyone would have done what he did if they had been in his same position.
In another act of bravery during the chaos of the shooting, Tucson resident Patricia Maisch, 61, grabbed the shooter's magazine before he could start shooting again.
"Somebody said 'Get the magazine!' so I got the magazine, and I was able to secure that," Maisch said. "That's what needed to be done."
Authorities said Maisch likely saved dozens of lives.
Also, a 20-year-old intern to Giffords, Daniel Hernandez, used his hands to apply pressure to a gunshot wound to her head while holding her in his lap. Hernandez, a University of Arizona student, is a certified nursing assistant.
Giffords was conscious and alert, Hernandez said.
"She wasn't speaking however, she was letting me know that she understood what I was saying by grasping my hand and squeezing when I would ask her to.
"I don't consider myself a hero," Hernandez said. "This is something that I've only done once and I hope I never have to go through again."
President Obama was expected to arrive in Tucson for a Wednesday memorial for victims of the shooting.
The Associated Press contributed to this report.