Man opens fire on employees inside a downtown package delivery facility before taking his own life.

A driver armed with a handgun opened fire at a United Parcel Service Inc package-sorting centre in San Francisco on Wednesday, killing three people before fatally shooting himself as officers closed in.

Wednesday’s shooting prompted a massive police response in a neighbourhood near the California city’s downtown area.

San Francisco Assistant Police Chief Toney Chaplin told journalists the employee killed three people and wounded two others before turning the gun on himself.

The man opened fire while the workers were gathered for their daily morning meeting before they were due to head out on their delivery routes, UPS spokesman Steve Gaut told the Associated Press.

The victims, like the gunman, were also company drivers, he confirmed.

Zuckerberg San Francisco General Hospital received the victims, spokesman Brent Andrew said, but he did not know exactly how many.

Police offered no explanation as to a possible motive for the shooting, but Chaplin told a news conference the shooting was not an act of terrorism.

Police recovered two firearms from the UPS facility, including the murder weapon, which they described as an “assault pistol”.

Auto shop owner Robert Kim said he heard about five to eight rapid gunshots.

The next thing he knew, he said, “a mob of UPS drivers” was running down the street screaming “Shooter, shooter!”

Uniformed UPS employees were later led out in a line by officers next to a highway. They walked away calmly with emergency vehicles nearby and gathered outside a restaurant.

“We are always saddened by the loss of life to gun violence. Any shooting is one shooting too many,” San Francisco Mayor Ed Lee said on Twitter.

Police recovered two firearms from the UPS facility, including the murder weapon, which they described as an “assault pistol”.

READ MORE: Top US politician Steve Scalise, aides shot in Virginia

The UPS facility, which employs about 350 workers in the city’s Potrero Hill area, was initially placed under a security lockdown as a precaution.

The gun violence erupted just hours after a different gunman opened fire on Republican politicians at a congressional baseball practice, wounding US Rep Steve Scalise of Louisiana and several others.

Gaut said the facility’s employees were dismissed from work once the lockdown was lifted and that most had since left the building. The company is providing trauma and grief counselling.

Video footage from the scene showed a massive police presence near the facility, with workers being escorted outside and embracing one another on the sidewalk.

“Our thoughts and prayers are with all those touched by this incident,” UPS said in a statement. A similar statement of condolence was issued by James Hoffa, president of the Teamsters union which represents UPS workers.

The San Francisco bloodshed came three years after a UPS employee shot and killed two of his supervisors before turning the gun on himself at a UPS distribution centre in Birmingham, Alabama. That gunman had recently been fired from the facility.

Gun laws in the United States rank among the most permissive of any developed country, with the right to “keep and bear arms” enshrined in the Constitution’s Second Amendment.

Efforts to tighten national gun control measures failed after mass shootings at an elementary school in Newtown, Connecticut, in 2012 and the nightclub shooting in Orlando.