They marched from City Hall to the Alameda County Administration Building in a rally that remained peaceful for more than four hours until nearly 8:30 p.m., when a small group of lingering protesters turned violent, smashing windows in several businesses and dozens of cars as rally organizers pleaded with them to stop. Officials briefly closed the 12th and 19th Street BART stations as a precaution and police arrested 18 people, mostly for vandalism.

Protesters planned the rally before Tuesday's arrest of former BART police Officer Johannes Mehserle to demand action in the officer's killing of Oscar Grant. After Mehserle's arrest, organizers decided to go ahead with the protest, in part to demand a murder conviction in the case.

Police estimated the crowd size during the march at about 1,000; protest organizers said there were twice that number.

Organizers with the Coalition Against Police Executions demanded the conviction, the resignation of Alameda County District Attorney Tom Orloff, the names of all BART officers present at the shooting, the creation of a citizen review board for BART and a network of healing centers for young people in Oakland.

"Oscar Grant's death will not be swept under the rug. ... We're here to turn the switch on so the (district attorney) has no problem seeing the facts," Bishop Keith Clark of Word Assembly Church in Oakland said at the rally at City Hall.

"We want you to prosecute this officer just like you would any murderer of any other youth."

Protesters began gathering at City Hall at about 4 p.m. and listened to speeches and pleas for nonviolence from voices as diverse as Oakland Mayor Ron Dellums and Oakland rapper Too $hort.

In brief remarks, Dellums asked the assembled crowd to emulate the famed Chinese protester of Tiananmen Square, who halted a tank column by standing passively in front of it.

"What he did he did without saying a word. He simply said he had had enough. Let this day be a loud and profound statement that we came together for justice with a passion for peace," Dellums said.

Dellums' comments were met by cheers and a few scattered boos. Too $hort's comments echoed the mayor's.

"It would be so sad if this turned out to be violent," he said. "Let's do this one right - everyone is watching."

Monitors in orange vests accompanied the marchers down 14th Street as they passed closed businesses boarded and shuttered for fear of vandalism. They passed David Gombogau, who was standing sentry outside his restaurant, Asian Grill.

"I hope everyone remains peaceful," he said, pointing at a window in his business that was shattered in last week's protest. "Businesses should not be attacked. It's OK for them to be doing this, but we just don't want them to attack us."

At the protest, Tiye Jones, 18, an Oakland native on break from Temple University, said she takes comfort in Mehserle's arrest but added that officials must review police conduct standards and continue the investigation into whether other officers acted inappropriately on New Year's Day when Grant was shot to death.

"Innocent people are being executed left and right. It's my duty to come out and speak against injustice. I take some comfort in the officer's arrest, but we have a lot more work to do. More arrests need to happen," Jones said.

Police and local businesses worried about the potential for violence despite organizers' calls for peace. Just one week earlier, a protest that started off peacefully ended violently with 105 people arrested and nearly 50 businesses damaged.

Wednesday, the same message boards that spread word about the rally also were filled with debate about how the protest should be conducted. Some people called for "bringing the ruckus" against what they called "legitimate targets" such as McDonalds and Foot Locker. Others called for nonviolent demonstration.

Many downtown Oakland employees left work early in anticipation of the rally, which began with speeches, Native American drumming and chanting and a light police presence.

Police Chief Wayne Tucker said every officer in the city's police department was available to monitor the rally, although officials hoped that the arrest Tuesday of Mehserle would cool the crowd's tensions.

Denise Geare of Pleasant Hill, an employee of the state attorney general who works in Oakland, said as she left her office around 5 p.m. that she hoped Mehserle's arrest would prevent a repeat of past vandalism.

"I was at People's Park. I'm all for civil unrest," she said. "But I believe there are some people who come out to be part of a violent mob, which is unfortunate."

Protesters began to trickle away from City Hall about 7 p.m., closely watched by police in riot gear. Tempers flared when a woman accused a man of stealing her cell phone; protesters shouted angrily as police arrested the accused man.

"It's a complex issue," said Richard Knee, a writer from San Francisco, as he watched the tense situation unfold.

"Of course you don't like to see violence, but there are times when nonviolence has not worked. It builds and it builds, and eventually it bursts forth in some rather unfortunate ways," he said. "Personally, I don't like to see it, but sometimes it's just avoidable."

Tension grew later in the evening as a group of about 100 refused to obey organizers' calls to go home, staying around 14th and Broadway as police reopened streets. Organizers kept the crowd under control, pulling protesters off cars and away from windows, until about 8:20.

At that point, a group of several dozen broke away, shattering windows in a bus shelter, Wells Fargo Bank and several businesses in the City Center Plaza shopping mall.

"There wasn't probably more than 10 people causing all the trouble," said Dan Lindheim, Oakland's acting city administrator. "The security for the demonstration did a great job, but at a certain point they couldn't deal with that. The good news is the police and the demonstration worked really well together. The bad news is you can't really control people who want to cause trouble and weren't part of the demonstration."

At 8:56, the shrinking crowd was milling near Broadway and 13th. Baton-wielding police moved in shortly after 9 p.m.

"The city of Oakland is now closed," police announced. "Please go home."

Chronicle staff writers Matthew B. Stannard and Charles Burress contributed to this story. E-mail Christopher Heredia at cheredia@sfchronicle.com.

This article appeared on page A - 12 of the San Francisco Chronicle