One person has been killed and at least three people injured after an active shooter opened fire near a polling station in Azusa, a suburb of Los Angeles.

Police said officers were pinned down by gunfire during the incident, which took place in a residential neighbourhood about 2pm local time but was not thought to be connected to the voting going on nearby.

There was one fatality, police said, and two people were transported to hospital by helicopter in critical condition, with one additional victim "down" at the "primary residence" where the incident took place. By 4.40am the police declared the situation under control, with all the officers involved safe and accounted for.

As shots rang out, people started to run inside the polling station at the Memorial Park and Slauson Middle School before officials closed the doors.

Elizabeth Hopkinson told NBC that she and her 9-month-old son were locked down at Slauson Middle School nearby, and that she was going to vote when the shooting happened.

She said she heard, "pow, pows, then a rain of fire."

She was one of 25 people locked in the gym, as well as a group of children. Another female voter, who spoke on condition of anonymity, told CNN that she was inside the polling station when she heard shots ring out. "At first, I thought it was construction but people came running into the room saying they see a guy with a bulletproof vest and a white shirt," she said.

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