Two Los Angeles police officers are recovering in hospital after being shot in what police are calling an ambush.
Video of the incident shows a figure approach the officers’ vehicle, before opening fire and running away.
Los Angeles Sheriff Alex Villanueva called the act “cowardly”. The suspect remains at large.
The Los Angeles County Sheriff’s Department has announced a $100,000 (£78,000) reward for any information leading to the gunman’s arrest.
Almost 40 US police officers were killed in 2020, FBI statistics show – eight of them were victims of an ambush.
Protesters shouted anti-police slogans and blocked the entrance to the emergency room where the two officers were being treated, police and witnesses said.
Two people were arrested, including a journalist who police said tried to interfere with the arrest of the other detainee and did not identify herself as a member of the press.
The reporter, Josie Huang, tweeted she had “thoughts and videos” to share, while her employer, NPR, said they were “appalled” by her arrest.
The officers involved in the shooting have not been named but were described as a 31-year-old woman and a 24-year-old man.
The female officer was shot in the jaw and arms and was in a critical but stable condition after undergoing surgery, authorities said on Sunday. The male officer was hit in the forehead, an arm and a hand and was described as alert.
“This is just a sombre reminder that this is a dangerous job. Actions, words have consequences and our job does not get easier because people don’t like law enforcement,” Sheriff Villanueva said.
Both candidates in this year’s presidential election weighed in on the shooting.
Sharing the footage of the incident, US President Donald Trump, tweeted: “Animals that must be hit hard.”
His Democratic rival, Joe Biden, said he was praying the officers recover.
“This cold-blooded shooting is unconscionable and the perpetrator must be brought to justice,” he tweeted. “Violence of any kind is wrong; those who commit it should be caught and punished.”