Dozens of people attending a permitted party at the San Leandro Marina scattered and ran for safety after someone fired a gun into the air, police said.
Police said the party grew from 200 guests to around 700 people due to social media. An event dubbed 'The Bay Area Linkup' was being held from 1 to 6 p.m. at the Marina, according to fliers posted on Twitter.
In addition to San Leandro police, Alameda County Sheriff's Office deputies and California Highway Patrol officers responded to the scene to assist with traffic control.
No other infromation was immediately available.