In Oakland, California, just east of San Francisco, peaceful protesters are being detained and arrested for being on the streets past the curfew.
There was a rally at a local high school, then they came downtown," said CNN Correspondent Dan Simon on the scene.
The Oakland Police Department said in a tweet that it had detained more than 40 people for violating curfew, and urged people to stay home.
The curfew went into effect at 8 p.m. local time, and will stay in place until 5 a.m. Tuesday morning.
It's not just Oakland: Further south along California, police in Los Angeles also arrested protesters who are out after curfew.
A small crowd was detained in Sunset Boulevard, in the middle of Hollywood, with their hands held by zip ties.
The Los Angeles police said on Twitter that "dozens" had been arrested for burglary after a drugstore had been looted earlier in the day. »