The demonstrations took on a new vigor this year, after the deaths of Michael Brown and Eric Garner -- two black men who were killed by white police officers.
More than 60 protesters partially closed the San Mateo Bridge in the San Francisco Bay area, the California Highway Patrol reported.
"It was peaceful, but they were blocking the bridge and were arrested," said Officer Damian Cistaro.
The disruption snarled traffic for miles.
Demonstrators also targeted transportation in nearby Oakland, shutting down the Bay Area Rapid Transit Coliseum station as part of a "Reclaim King's Legacy" march.
Seventh-grader Kai Jones said he thinks it's important to protest because "young men my age and older are getting killed, and it's not fair," he told CNN affiliate KGO
The day started very early with a 5 a.m. protest outside the home of Oakland Mayor Libby Schaaf, which was just fine with her.
"I woke up this morning knowing it's great to be an American, that we are a city that allows people to peacefully assemble and to protest," she said.
In Seattle, the situation was dicier as demonstrators danced on roadways, challenging police to corral them.
One police officer suffered a serious leg injury, responding to protesters, authorities said.
Organizers said civil disobedience is a way to bring about change.
"People are putting their lives on the line to make a statement that black lives do matter," Monica Avery told affiliate KOMO
. "And there is a national call ... to start moving our lives and putting our bodies on the line to move the movement against police brutality."
Nineteen demonstrators were taken into custody, KOMO reported.
In Boston, hundreds protested police brutality and called for social justice.
The 4-mile walk to Boston Common touted the familiar messages of recent protests -- "Black lives matter," "Jail killer cops," "Remember Ferguson," and the thought-provoking "Who will be our next King?"
In Minnesota, demonstrators descended on the state Capitol in St. Paul. Along the way, they shut down traffic briefly on Interstate 94.
U.S. Rep. Keith Ellison praised the fight for racial equality.
"They're putting their lives on the line," he told affiliate WCCO
. "And sometimes they disrupt traffic; they make a lot of noise. They're trying to get our attention."
But it wasn't all protests and demonstrations.
In Los Angeles, tens of thousands watched the 30th annual Kingdom Day Parade, with 3,000 taking part.
This year's theme: "Love & Respect: Let It Begin With Me."
The parade was a celebration of King's life, complete with marching bands and some advice from the grand marshal on how all people can get along.
"If you want to be successful with anything, take a peaceful approach that we are all together," Herb Wesson, a City Council member and grand marshal, told affiliate KTLA
. "What's good for one group of people is good for all groups."
The observances come as The New Yorker unveiled a poignant magazine cover, combining the historical with current.
Called "The Dream of Reconciliation," the cover shows Martin Luther King Jr. marching arm-in-arm with Brown, Garner, Trayvon Martin and NYPD Officer Wenjian Liu. Liu and fellow Officer Rafael Ramos were gunned down in their patrol car last month.
"It struck me that King's vision was both the empowerment of African-Americans, the insistence on civil rights, but also the reconciliation of people who seemed so hard to reconcile," artist Barry Blitt said
"In New York and elsewhere, the tension between the police and the policed is at the center of things," he said. "It is hard to believe things would have got as bad as they are if he was still around today."