OAKLAND — A clash between police and protesters Saturday night ended with six protesters arrested, windows broken in a Starbucks coffee shop and a handful of police cars, and Occupy Oakland organizers promising more action opposed to police violence.

A crowd estimated at between 200 and 300 people began marching around 8 p.m. at Frank H. Ogawa Plaza, which has served as a rallying point and occasional camping ground for Occupy protesters.

Occupy organizers said it was mostly a peaceful crowd with a small number of agitators causing trouble, while Oakland Police Department spokeswoman Officer Johnna Watson said it was “mainly the more hard-core agitators” and “the most hostile, aggressive crowd, not like we’ve seen with the Occupy group.”

A flier promoting the march had named the event, “March Against Repression: (Expletive) OPD.” As protesters made their way through downtown Oakland, some allegedly broke police squad car windows as well as those of a Starbucks coffee shop. Watson said the tires of a news van were slashed and that several glass bottles were thrown at police officers.

Protester Diane Reiner, 59, said the “huge majority” of people there were nonviolent — some of them having brought out a bubble machine to blow bubbles signifying their peaceful intent. She said the crowd marched in anger about police actions against Occupy Wall Street camps and events across the country.

“The rising importance of police conflict and government crackdowns on citizens trying to exercise their First Amendment rights is becoming a much higher priority for people inside the Occupy movement,” Reiner said. “For people like me, who originally joined to focus on national broad financial issues, media and politicians being controlled by corporate money, now this direct oppression by extreme policing is becoming a higher priority.”

Spencer Mills, a member of Occupy Oakland who also acts as a self-described citizen journalist during marches and releases live, streaming video under the Twitter handle @OakFoSho, said he was disappointed with both sides of the conflict.

“I did see a couple people that did throw stuff from the back of the crowd,” he said. “When that happened, the other protesters got really upset. Folks were trying to find out who did that stuff, because the people up front would have felt the wrath of OPD if they decided to do anything about it.

“It disappoints me greatly,” Mills added, saying he wants Occupy events to remain peaceful. “They endanger the lives of a lot of people when they do that.”

However, he said, he felt police were also overly aggressive, citing a video posted on YouTube of what appears to be a skirmish between several officers and a girl on a bicycle, under the title, “OPD slams woman and her bike to the ground, beat her and chase her down the street!”

Watson said Sunday she hadn’t seen the video and therefore declined to comment on it. She said there were no injuries reported either to officers or protesters.

It was not clear whether anyone had called to file a use-of-force complaint, but Watson said people can call 510-238-3161 any time of day or on weekends to initiate complaints with internal affairs.

Protesters were arrested on suspicion of assault on a police officer, resisting arrest and possession of explosives. Police said one person had a device that was the equivalent of a quarter stick of dynamite — not an M-80 firecracker, Watson said, but a 6-inch-long explosive.

“I’d never seen anything like it before,” she said. “That’s not something that’ll blow your thumb off; that’s something that could easily kill you or take a body part off.”

The march ended by about 11 p.m. near the Oakland police headquarters at Seventh Street and Broadway, but a smaller crowd remained in the area until at least 1 a.m., Watson said.

Organizers promised future marches and other events, though nothing specific was planned as of Sunday afternoon.

Staff writer Joshua Melvin contributed to this report. Contact Sean Maher at 925-779-7189. Follow him on Twitter at @OneSeanMaher.