A Rogers Park man has been charged with a vicious, unprovoked attack on a 91-year-old man in broad daylight on a busy Chinatown street Tuesday afternoon.

Alexander Taylor, 45, faces three felony counts of aggravated battery for the beating that happened just after noon in the 2400 block of South Wentworth, according to Chicago police.

The elderly victim, identified by family as Yen Jit Wong, remained in intensive care Wednesday at Northwestern Memorial Hospital, according to one of his grandsons, Jacky Lau.

Chicago Police Sgt. Michael Bazarek happened to be driving down the block looking for a cup of coffee while on break from a work seminar when he witnessed the bizarre encounter.

The sergeant said he noticed Taylor acting erratically as he strode down the sidewalk, frightening other pedestrians by “making wild gestures and pretty much speaking gibberish,” Bazarek said at a Wednesday press conference.

“Something was clearly wrong with him. I feared something was going to happen,” Bazarek said.

That’s when Taylor reared back and, “without provocation,” punched Wong in the head, knocking him unconscious on the ground, Bazarek said.

The sergeant told one of the dozens of people on the block to stay with Wong, as he got back in his car to follow Taylor, who kept calmly walking toward 26th Street and LaSalle, Bazarek said. The officer boxed him in with the vehicle and stayed with him until more officers arrived to put him in custody.

“It’s shocking but it’s not surprising,” Bazarek said. “I’ve been doing this for a long time and I’ve seen this before.”

Wong’s head hit the pavement, causing bleeding on the brain, his family said. He also suffered a broken collarbone. Police said his condition was stabilized.

“He’s 91 years old. He’s totally defenseless,” Lau said. “What kind of monster does this?”

Taylor is being held without bond at Cook County Jail, according to the Cook County sheriff’s office. His next court date was set for Tuesday.