And it begins:

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

Los Angeles county sheriff captain Jeff Scorggin said authorities were dealing with one suspect who was heavily armed.

The shooting took place in a residential neighbourhood about 2pm local time, and police said that four gunshot victims were being treated for their injuries.

The extent of their injuries is not known as police said the situation was not "contained".

Officer Jerry Willison of the Azusa police department said the situation as "very volatile and critical".

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. The shots did not happen inside the polling station.

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. As of right now, they just have us in the voting room and are trying to keep us calm."

A helicopter has transported at least two victims to hospital.

Police have urged residents to stay indoors. The suspect reportedly was shooting at police but it is unclear if any police officers have been injured.