Shots were fired at a gay pride event in San Francisco, leaving one man injured, city police said.

The incident was believed to have occurred when several groups of men unrelated to the pride event "got in a verbal argument near or inside the venue," the San Francisco Police Department said in a statement.

The pride spectator who was shot was believed to be "unrelated to the dispute", SFPD officer Carlos Manfredi told AFP.

Police said the incident escalated "when one of the subjects pulled out a gun and fired several shots. One shot struck a pride spectator in the arm."

Several suspects have been detained, the statement added.

Police were alerted to the shooting, which occurred in the United Nations Plaza area, just after 6 pm.

The 64-year-old victim was transported to San Francisco General Hospital where he is in stable condition, police said.

The city's gang task force, crime scene unit and night investigations unit are investigating the incident.

The shooting comes just one day after the US Supreme Court made same-sex marriage legal throughout the nation.