Dozens of people were arrested overnight after demonstrators poured onto the 101 Freeway in Los Angeles, backing up traffic for miles, in protest of the presidential election of Donald Trump.

An estimated 3,000 people turned out at City Hall downtown before the demonstration moved down the road and eventually onto the freeway, according to Los Angeles police Lt. Chris Ramirez.

The protesters stayed there for most of an hour, with drivers sitting and waiting and many getting out of their cars, the Associated Press reported.

Though demonstrations were largely peaceful, protesters grew more aggressive and violent as the evening wore on, Ramirez told KPCC.

Ramirez said there were reports of protesters throwing rocks and bottles at police, along with vandalism of some city buildings and at least one police car.

L.A. Mayor Eric Garcetti issued a statement saying he was proud that protests had remained mostly peaceful, but he urged everyone to put safety first.

“I understand that the results of Tuesday’s election are painful for many of us, and this kind of engagement can be a meaningful part of the healing we need after such a long and divisive campaign,” Garcetti said in a written statement.

"But walking and throwing objects onto freeways is dangerous for pedestrians and drivers — and it puts a heavy burden on people just trying make it home to their families or get to work safely."

Once the crowd moved onto the freeway, the LAPD and the California Highway Patrol issued an order to disperse and eventually began arresting anyone who failed to comply, Ramirez said.

Photos from the scene showed officers in full tactical gear, with protesters squaring off against police. About 28 people were arrested.

Ramirez said people have the right to protest, as long as it doesn’t harm anyone else or threaten public safety.

“When it’s unannounced and it goes onto public highways like the freeway, where it can be hazardous not only to the motorists but to the public, that’s when it becomes illegal,” Ramirez said.

The anti-Trump demonstrations started Wednesday afternoon and ran into the night, with protesters shouting “not my president” and “immigrants welcome here.” At one point, a giant effigy of Trump’s head was burned on the steps of City Hall.

“My marriage might not be legit anymore, because of him. My health care might not be accessible anymore because of him,” said Boyle Heights resident Jayden Bedard. He told KPCC he’s worried about the effect the Trump presidency could have on the Supreme Court and Congress.

The demonstrators were among thousands who took to the streets in protest of Trump across the country, from New York to Texas to much of the West Coast, according to AP.

About 7,000 people gathered in downtown Oakland, while another big crowd marched up San Francisco’s Market Street.

Protests in Santa Ana also turned rowdy. An estimated 650 demonstrators marched to Bristol Street and McFadden Avenue, according to the LA Times:

A brick was thrown into a police cruiser, two other vehicles were damaged and two businesses were vandalized, said Santa Ana police Cmdr. Phil Craft.

Officers fired bean bags to disperse the crowd, and 10 people were arrested, he said. The Orange County Sheriff’s Department assisted police with the crowd, officials said.

Ramirez told KPCC he was not aware of any other protests planned for Thursday, but he said in L.A. demonstrations can pop up without much notice.

This story has been updated.