At least nine fires were set in the city - four to buildings, four to cars and one in a dumpster, fire officials said.
Dozens of people were seen taking merchandise from stores in the shopping district.
People with arms full of boxes were walking quickly away from the area. In some cases, there appeared to be organized groups of people loading cars full of merchandise.
At least one vehicle was seen with out-of-state license plates and another had its license plate covered.
The looting was happening just blocks away from a large protest on Montana Avenue that was mostly peaceful, although many in the crowd did not disperse when ordered to by police.
Protesters staged a sit-in, with police setting up a skirmish line around the protesters.
Santa Monica officials emphasized that they did not believe most of the looters were associated with the protest.
Still, authorities eventually declared a 4 p.m. curfew in an effort to disperse the protest crowd.
Hours later, there were still hundreds out on the street and police began efforts to send people home.
"We are making arrests as we speak," said Santa Monica police Chief Cynthia Renaud at a 7 p.m. press conference. "The 4 p.m. curfew is in place."
The westbound 10 Freeway was closed at Bundy and other freeway offramps leading into Santa Monica were closed in an effort to limit people from entering the city.
Police began deploying tear gas and called in help from the National Guard to disperse the crowd. Officers were also seen firing nonlethal rounds into the crowd. They also fired nonlethal rounds into a building, where someone was throwing golf balls and other objects at officers.
Around 5 p.m., heavy smoke was pouring out of building near Fourth Street and Santa Monica, as looters were seen running out of a nearby doorway. The fire department arrived on scene within minutes.
A car was also set on fire. As heavy smoke poured into the air, people stood nearby and took selfies with the flames in the background.
Standing up to looters
But even among the scenes of chaos and looting, there were stories of heroism and people standing up against the looters.
One Santa Monica jewelry store owner said a young woman parked herself on a bicycle in front of his store and may have helped keep troublemakers away.
Bill Bubar of Bubar's Jewelers, a business in Santa Monica for 75 years, says an organized mob tried to break into his store.
"Basically it was a mob out of control," he told ABC7. "They were driving in the middle of the street. They jumped out of cars."
He said one person in the group came over to the store with a baseball bat and tried to smash the door and push the gate in. But the person couldn't get through the gate, he said.
After that first group left, Bubar said, a young woman on a bicycle that he didn't know came by and stayed in front of the store for two hours helping to ward off additional attempts.
"With all of this commotion she stood here and literally I think they left it alone because she was standing in front of the door," Bubar said. "She was really the hero."
He noted that the store's actual jewelry was locked in a safe that the looters likely wouldn't have been able to get into, but they could have done a lot of damage to his equipment, collectibles, mirrors and display cases.
On Saturday, multiple communities in Southern California saw extensive protests, with some of them resulting in violence, including fires and looting. Police struggled to maintain control of the crowds, and the National Guard was ultimately called in overnight to help maintain control in Los Angeles. Almost 400 people were arrested in Los Angeles.
Looters seen filling cars with merchandise in Santa Monica