OAKLAND -- The sound of conversation and soft laughter wafted through Frank H. Ogawa Plaza on a mild Saturday morning. Mothers, fathers, sons and daughters wrote and colored on large placards. It could have been a play date, or a child's birthday party.

It was billed as a "Moms March," a peaceful gathering to urge a federal investigation into the death of Florida teenager Trayvon Martin. The sign-making was followed by a short walk to the Oakland Federal Building, where the Moms March met up with the Student March For Peace and Justice. Together the groups staged a nonviolent demonstration as part of the Rev. Al Sharpton's "Justice For Trayvon" National Day of Action.

"I'll show you what brings me here," said Sweet Grass Longhouse, a retired teacher and grandmother from Berkeley as she pulled out a photo of her grandson Canon Christian Jones II.

"He was shot and killed in Alabama (in 2007) because there are just too many guns," she said.

Her grandson, an 18-year-old African-American Berkeley High School graduate, was a freshman at Tuskegee University. He was walking to a convenience store on a Sunday night when he was gunned down.

"This is a concern," Longhouse said. "The president spoke to it yesterday, about the violence in the African-American community itself and especially the young men."

Not far away, Oakland resident Marcelle Poulos was coloring a placard as her 8-year-old son, Fred Hughes, looked on.

"I feel frustrated with the outcome of the (George Zimmerman) trial, and I just feel like it's important that people exercise their right to protest," she said. "I think it's good to set this example for my son as well, that he knows about this and that not everybody has the same privileges. He should be aware of that."

The Moms March participants walked and chanted down 14th Street, then turned left at Clay Street. Combined with the student demonstrators, they comprised a group of about 200 people.

The first speaker there urged the crowd, "Let's not let any violence take place in Oakland today or tonight. No graffiti. No broken windows."

"I want the action to be peaceful," Longhouse said. "Last year, (the Occupy protests) began peaceful. Anarchists, they move in on things, and then you have two things happening."

Anarchy wasn't a problem early in the demonstration at the federal building. But hearing was. A balky bullhorn and a pair of TV news helicopters drowned out the speakers early in the demonstration.

The crowd chanted, "Peace on the street!" And later, "Respect our city!" Some cars driving past the scene honked their horns in support.

Approximately two dozen uniformed Oakland police officers stood on the periphery of the crowd. One, with the help of a telescoping pole, took video of the scene.

The civility waned when Oakland Mayor Jean Quan tried to address the crowd. The mayor was booed and heckled by a small handful of onlookers, who were angrily shouted down by others.

Los Angeles and San Francisco were among other California cities to hold rallies associated with the National Day of Action. Sharpton appeared at a rally in New York, where Sybrina Fulton, mother of Trayvon Martin, spoke.

Contact Gary Peterson at 925-952-5053. Follow him at Twitter.com/garyscribe.