Witnesses to the stabbings at a mall in a Sydney, Australia, on Saturday described a scene of terror as shoppers fled from the knife-wielding man or huddled in stores as panic spread through the shopping center.

Some shoppers hid inside as alarms blared. Others ran out, screaming as they passed by bodies on the floor.

When Gavin Lockhart, 37, saw people running as he sat inside a coffee shop at the mall, there was a moment of confusion. “Is it a celebrity?” he first thought. “Is it because of a gunman?”

Then he fled when he heard, “He’s got a knife! He’s got a knife!”

He followed the coffee shop’s owner, Michael Dunkley, 57, who also brought his wife, who was cooking, and two baristas into a staffroom where they could lock the door. Mr. Dunkley said afterward that just one thought was in his mind when the screaming began: “I have to get my wife and staff to safety.”

Mr. Dunkley left the room to try to chase down the attacker, whom he described as a thin man with a beard and short hair, wearing dark green pants and a green jersey.

Then, Mr. Dunkley recounted, he saw a police officer attempt to stop the assailant. When the officer told the man to put his knife down, he lunged toward her with his weapon, the cafe owner said.

“He didn’t say anything,” Mr. Dunkley said. “He seemed determined.”

The officer then shot the attacker on the fifth floor of the mall, on a walkway near a phone store and a clothing alteration shop, Mr. Dunkley said. When the attacker fell, the officer immediately began administering CPR on him, the cafe owner said.

“In this country, this stuff shouldn’t happen,” Mr. Dunkley said. “People come here because it’s safe.”

Andrew Reid, 44, had been shopping for a bed when he heard that people had been stabbed at the mall and shoppers were told to evacuate. Many of the stores were in lockdown, but after seeing people lying on the floor, bleeding, he said, he used his training as a lifeguard to help two women.

One had a wound in her back, he said, expressing outrage that the attacker would stab someone from behind. “It’s so cowardly,” Mr. Reid said.

“We grabbed some clothes out of the clothes store, trying to stop the bleeding,” he added.

About 30 yards away, the second woman lay unconscious, he recounted. He ran over to find a deep wound on her chest just above where he needed to do compressions, which made CPR difficult.

“There was a lot of blood around her,” he said. “I honestly don’t think she made it.”

Even hours after the attack, witnesses were struggling to process what had just happened.

In his 20 years working as a lifeguard at nearby beaches, Mr. Reid said, he had mostly dealt with drownings. While he had experience with wounds, he said he did not usually treat several stab victims in succession. He said that he was horrified, but tried not to be affected while trying to save people’s lives.

“You just detach emotion from that sort of stuff,” he said. “You just got to.”

Mr. Lockhart, who said he had seen the officer shoot the attacker, sounded dazed. “The one positive I’m looking at is the police officer probably saved my life,” he said.