OAKLAND -- A protest at City Hall that surprised both the Police Department and top city officials caused more than $12,000 in damage to public buildings and thousands more to private businesses, city officials said Monday.

A day after about 200 protesters roamed downtown streets with no consequences, smashing windows of businesses and city offices, city officials were still trying to explain why no one was arrested and why police were unprepared.

The protest was billed on Facebook as an anti-imperialist rally and march, timed to coincide with the 11th anniversary of the start of the U.S. war in Afghanistan.

But the Police Department had no idea the march was going to occur, and by the time officials realized a large gathering was forming and vandalism had begun, it was too late to call in off-duty officers for extra support, said police spokeswoman Johnna Watson.

"We did not have any information that we had 200 to 300 protesters on their way here," Watson said. "The march presented itself, and in that march, they went very quickly doing their vandalism.

"That is very difficult from a law enforcement perspective," Watson continued.

Protesters gathered about 6 p.m. at Frank H. Ogawa Plaza for a rally, then left on foot about 7:15 p.m. heading north on Franklin to 20th Street before circling back to City Hall, police said.

About an hour later, protesters had left a swath of vandalism in their wake as they broke glass front doors and windows at two city offices and about nine private businesses, including the Oakland Tribune.

Two windows were broken at the newspaper's office. Protesters also targeted three bank branches, the offices of Kaiser Permanente, AC Transit, Fox Theater, Sears and the Oakland Scientific Facility, which houses computers and data storage for Lawrence Berkeley Laboratory. Banks were closed for Columbus Day on Monday.

Protesters also slashed tires and broke windows on a couple of cars.

Watson said police initially believed the protesters who were gathering in front of City Hall on Sunday night were part of Afghans for Peace, a group dedicated to ending the war in Afghanistan. That group had previously asked for and was granted permission to hold a rally at City Hall, and Watson said it has a history of holding peaceful protests.

However, she said, the Afghans for Peace group was joined by another group that caught the Police Department by surprise.

In addition, unlike Friday and Saturday nights, when the department placed extra officers on patrol in anticipation of possible rallies, Watson said Sunday night's patrol was staffed at a normal number and thus was unable to respond to the brisk vandalism.

"By the time you could even gather enough officers to address what is going on, it's over with," Watson said.

Asked what would prevent Sunday night's group or another group from using the same tactic on another night Watson said, "We will do the best we can to avoid that."