Business owners along Oakland’s Auto Row cleaned up and took stock Saturday after vandals rampaged through the area, shattering store windows and smashing car windshields in a May Day protest whose ferocity took police by surprise.

Oakland police did not call for backup help until 8:44 p.m. Friday, spokeswoman Johnna Watson confirmed — some 15 minutes after vandals in a crowd of several hundred people had begun breaking glass. But by then it was too late for enough officers from other law enforcement agencies to get there and prevent the damage.

About a dozen people were arrested in connection with the destruction, which Oakland Police Chief Sean Whent called among “the worst that we have seen in recent years.”

Hundreds of demonstrators marched through downtown Oakland, causing mayhem and damaging scores of autos in car lots on Broadway, including more than 50 vehicles at a Hyundai dealership.

Vandals also smashed the windows of Wells Fargo and other banks, as well as stores, while others yelled, “Don’t harm small businesses!”

Whent acknowledged Saturday that the crowd was larger and “angrier than we expected,” and that the timing of the call for help from outside agencies — after the vandalism had begun — “minimized the effectiveness of our mutual-aid response.”

Mayor Libby Schaaf, who had said beforehand that the city would not tolerate violence, conceded that “we did not do as good of a job as we should have protecting property.”

The vandals were among a large number of people who set off from City Hall on a march through downtown Oakland at 6:30 p.m. to protest police brutality. Protesters shouted, “Baltimore, we got your back!” — a reference to the death of a Baltimore man fatally injured in police custody and the announcement Friday that Maryland’s state prosecutor would charge six police officers in connection with the death.

At first, the vandals set a trash can on fire and threw bottles at buildings. Police in half a dozen cars and vans followed without intervening. But by about 8:30, the vandals had taken up more extensive acts of destruction. Only afterward, when police put out the call for aid from surrounding agencies, did dozens of police in riot gear show up to try to contain the protesters.

One of the protesters’ main targets was Auto Row, where the windows of more than 50 new Hyundais were smashed at Broadway and Hawthorne Avenue. Rufus Keller, the dealership general manager, said vandals had spray-painted another 10 used cars in a lot across the street.

At nearby American Auto Upholstery and Glass at 3080 Broadway, vandals smashed eight windows. Joe Abraham, who owns the store, spent Saturday morning cleaning up shards.

“The city has to do something about it,” Abraham said. He estimated it would cost $8,000 to replace the glass.

“We are small businesses. We can’t afford to replace windows — we barely pay our bills here,” Abraham said. “What are they gaining by damaging windows and cars?”

One block north at Broadway and Piedmont, the hoodlums damaged at least eight cars and smashed showroom windows at a Honda dealership. Along Broadway, where several shops and a building under construction had been hit during the pandemonium, some were already boarded up.

Nearby resident Carol Beck, 50, came to view the destruction.

“I'm tired of it,” she said. And Schaaf, she said, “did nothing to stop this.”

“There is a black cloud that hangs over Oakland,” Beck said. “It’s very sad.”

On Broadway near 30th Street, 66-year-old Bruce Burrows looked at the eight broken windows in a vacant building that used to house a car dealership. He said the property has been in his family for years and was the site of the city’s first dealership — opened by his grandfather a century ago.

“I've been on this property since I was 8 years old,” Burrows said. “I don’t think the city has control. ... It goes back to City Hall.”

At a news conference Saturday morning, Schaaf said, “A very small group of people with rocks in their pockets and malice in their hearts disturbed what was otherwise a positive day in Oakland.”

The mayor described those involved in the unrest as “cowardly people that use the cloak of night and large crowds to commit this type of vandalism.”

Whent, who was also at the news conference, said police had arrested about a dozen people on suspicion of a variety of crimes, including burglary, vandalism and failure to disperse after officers declared an unlawful assembly.

An officer suffered a cut to his leg, Whent said. Police used teargas once during the protest, he said.

“We do tolerate marching. We do not tolerate vandalism,” Whent said. “We do absolutely attempt to make every one of those arrests that we can.”

Whent and Schaaf said police may make more arrests after reviewing surveillance-camera footage. “We have a good track record of arresting people and holding them accountable after the fact,” the mayor said.

Hamed Aleaziz and Henry K. Lee are San Francisco Chronicle staff writers. E-mail: haleaziz@sfchronicle.com, hlee@sfchronicle.com Twitter: @Haleaziz @henryklee