A 59-year-old San Francisco man was on life support Saturday, a day after he was assaulted in broad daylight in downtown Oakland trying to defend his son's honor in what Mayor Ron Dellums called "a brutal and random attack."

Tian Sheng Yu was punched once in the mouth by an unknown assailant and fell to the ground, hitting his head, said his son, Jin Cheng Yu, 27, who witnessed the blow.

"It happened so fast," the younger Yu said Saturday afternoon in front of his home in San Francisco's Portola district, where he stood still wearing pants splattered with blood, his left eye badly swollen from the unprovoked blow. His father was listed in grave condition and was in intensive care at an Oakland hospital.

Jin Cheng Yu, a recent graduate of UC Davis, said that he and his father went to the 1800 block of Telegraph Avenue in Oakland around 3 p.m. to check out coins in a jewelry shop when the attack occurred.

Tian Sheng Yu let his son out of the car while he went to look for a parking place. Jin Cheng Yu started walking toward the shop when one of two young men or teenagers walking toward him slugged him in the eye "for no reason." Stunned, he caught up with his dad and told him what had just happened.

"My father wanted to know why I didn't fight back," he said. "He took me to go look for them."

He said they found the suspected assailant and his companion around the corner.

"My father asked them in Chinese - his English isn't very good - 'Why do you beat my son?' They didn't say anything. Then one of them punched my father in the mouth. He fell backwards to the ground."

The suspects then turned to the son. Jin Cheng Yu said they punched him "four or five times." He said he tried to hit back, but kept missing. Jin Cheng Yu had never been in a fight before.

Witnesses called 911. One of those callers was Jean Van Fleet. She saw the attack through the front window of her bookstore, Bibliomania, across the street. A passer-by yelled into the store for her to call police, and when Van Fleet looked outside she thought she saw three young men beating up someone else.

She was on the phone with the 911 dispatcher for only two or three minutes she said, and when she turned back to the street, the young men were gone and Yu was on the ground. His face was bloody and his body was convulsing.

"I'm so sad he was beaten so badly," Van Fleet said Saturday, adding that she never saw the faces of the young men involved in the attack. "I kind of wish I'd gone over there with a broom and yelled at them all to leave."

Tian Sheng Yu never got up from the sidewalk. He was rushed by ambulance to a nearby hospital. His son said his chances for survival are slim.

"It's all my fault," he said after a sleepless night keeping vigil with his mother at the hospital. "I shouldn't have ever told my father I was punched."

A motive for the attack has not been determined, according to police. Jin Cheng Yu said he was not robbed.

Oakland police spokesman Jeff Thomason said the attack was unprovoked.

"There was no warning, no explanation," Thomason said. "The father did not walk up in a threatening manner. He just wanted to know what happened. Any father would want to do that."

It was not the first time the family fell victim to crime. In January, Tian Sheng Yu's wife was robbed on Third Street in San Francisco, her son said.

The family emigrated from Beijing in 1998. Tian Sheng Yu started a painting and remodeling business. His only child graduated from UC Davis with a degree in biochemistry and had plans to go back to school.

"Now I don't know what I'm going to do," said Jin Cheng Yu, who lives with his parents in a modest house on a dead-end street with sweeping views of the East Bay. "I have to get a job and support my family. I can never forgive myself for what happened."

Dellums issued a statement Friday, condemning the assault.

"The brutal and random attack this afternoon in broad daylight on two innocent men was senseless and outrageous," Dellums said. "Our hearts and prayers are with the father, now on life support, and his loved ones who are also suffering."

The mayor said police have stepped up patrols in the uptown and downtown business districts, and "they will pursue every lead until those responsible for this violence are apprehended."

Police released surveillance video and still images of the suspects and asked for the public's help in identifying them.

Thomason described the first assailant as a black male teenager with a medium to dark complexion, short hair, 5 feet 7 inches tall, weighing 120-130 pounds and wearing a black "Raiders-like jacket." He described the second suspect as black male 17 to 20 years old with a medium to dark complexion, short hair, 5 feet 9 inches to 5 feet 10 inches tall, weighing 150-160 pounds and wearing a maroon shirt.

The assault occurred on an open stretch of sidewalk next to the Fox Theater, beneath a towering palm tree. The 1800 block of Telegraph Avenue is in many ways an up-and-coming neighborhood, area business owners said, with new restaurants and bars opening amid some of the older pawn shops and cash checking storefronts.

Van Fleet said that in 15 years of running a used book business on the corner of Telegraph and 19th Street, she's never felt unsafe. Sometimes troubled people will walk into her store and yell or make a scene, but she's always been able to handle them.

A few blocks down the street, Warren Taylor said he wold have moved a long time ago if the neighborhood weren't safe. His psychology office is just a couple of blocks from where the attack took place.

"It's a good neighborhood," Taylor said. "This was really an unfortunate incident. But it could have happened anywhere."

John Hopes, a city worker who cleans and sweeps the streets in the neighborhood six days a week, said he's never seen any kind of violence in the area.

"Further downtown, sure," he said. "But around here it's rare. People come here and do their business and just pass through. Actually, they're really nice to me, always saying hi. It's really sad what happened."

Oakland homicide investigators were called in to help with the probe because of the severity of the assault, according to Oakland police spokeswoman Holly Joshi. Anyone with information is asked to call (510) 238-3821.