The incident occurred at about 5pm local time yesterday in Peregrine Park in northern Sacramento.
Officer Doug Morse told the Sacramento Bee that attackers walked up and opened fire, then fled in a car.
A man aged in his 20s died at the scene. Police say he is believed to have been a gang member, but his name has not been released.
Mr Morse said six other people - including a seven-year-old - received non-life threatening injuries.
The group had gathered to celebrate a child's first birthday.
COMMENT RULES: Comments that are judged to be defamatory, abusive or in bad taste are not acceptable and contributors who consistently fall below certain criteria will be permanently blacklisted. The moderator will not enter into debate with individual contributors and the moderator’s decision is final. It is Belfast Telegraph policy to close comments on court cases, tribunals and active legal investigations. We may also close comments on articles which are being targeted for abuse. Problems with commenting? firstname.lastname@example.org