San Francisco Police Chief Greg Suhr said new estimates for the Gay Pride Parade on Sunday have 1.5 million people making their way to Market Street.
He said it will rival and possibly top the San Francisco Giants World Series parade.
"Even though the World Series was huge, this could quite possibly be larger in light of the decision that just came down from the Supreme Court - but it will be just as happy as the World Series," Suhr said.
He said his officers will be out in force, just in case to ensure a peaceful event.
The increased crowd follows this week's historic court decision to strike down Prop 8. The normal estimate for the parade is a million people.
Police said safety at the event will be a top priority.
Police said they are urging Pride participants to be cautious with strangers and to report suspicious situations.
Pride celebrations got an early start with Wednesday's Castro District rally that drew thousands of people to Castro Street.
Many San Francisco hotels are sold out for the weekend.
The Pride Festival happens on Saturday and Sunday and includes more than 20 stages throughout the Mid-Market area with musical acts, performances and countless dancing opportunities.
The 18th display of the Pink Triangle will go up on Twin Peaks on 7 a.m. Saturday morning before a 10:30 a.m. ceremony with city and Pride officials.
Saturday night the Sisters of Perpetual Indulgence will host an alcohol free party in the Castro.
Sunday's parade is the main event and starts at 10:30 a.m. and winds down Market Street ending at the Civic Center.
Merchants have been given a "heads up," Castro/Upper Market Community Benefit District executive director Andrea Aiello said, and extra staff has been called in. Aiello said she expects crowds to "stream into the Castro after the parade" but the effect of the Supreme Court rulings will be felt throughout. "All of San Francisco is buzzing," she said.
Motorists are advised to expect delays and crowded streets along Market Street, downtown and Civic Center areas and the Castro District.