CASTRO VALLEY -- What started out as a small march to show support for gay rights has grown into the full-blown Castro Valley Pride festival, celebrating its third anniversarynext Saturday.

This year's festival will feature live music, a DJ, more than 40 vendors and gourmet food trucks and is expected to draw hundreds to Castro Valley High School's parking lot.

That's in contrast to 2011, when teenagers Frankie Canto, Johnna Murch and Holly Zeitz expected about five people to join them, Zeitz said.

"We thought we would run down to Castro Valley Boulevard and be crazy kids showing support for gays," she said. "We had been to Prides in San Francisco and Oakland, and we thought it would be cool to have something in our own town showing that LGBT youth are welcome here."

They posted the event on Facebook and invited their friends, who invited their friends, she said. One of those who saw the Facebook post was activist Billy Bradford, of Castro Valley, who volunteers with national gay rights organizations Marriage Equality and Get Equal.

"Because I had experience with national groups organizing events, I contacted them and said that I would help them," Bradford said. "It got really big really fast. We thought there would be 12 to 20 people showing up and we would walk down Castro Valley Boulevard. Then it was 50 people saying they would attend, then 75."

When the number hit 150, they were told the march would need a permit, but there was not enough time to get one. Bradford contacted the Castro Valley school district, which allowed them to hold a rally at the high school.

"We had a tiny stage and a small speaker," Bradford said. They used an upside-down milk crate as stage stairs.

When about 300 people showed up, "We said, 'Wow, it looks like we have a Castro Valley Pride,'" Bradford said.

Since then, the group has become better organized, and different people have become involved. Canto and Murch have moved on, and Zeitz is not an active organizer, though she plans to attend.

This year's festival will have a larger stage, a better sound system, a DJ, musical performers, bathrooms and security. Food Truck Mafia is sending two gourmet food trucks. The San Francisco Giants, Oakland A's and San Jose Sharks have donated items for a raffle, and 40 vendors and groups will set up booths.

Speakers include state Senate Majority Leader Ellen Corbett, U.S. Rep. Eric Swalwell, Assemblyman Bill Quirk, Alameda County Supervisor Nate Miley, San Leandro Mayor Stephen Cassidy and the Rev. Katie Kandarian-Morris of Starr King Unitarian Universalist Church.

And festival organizers have been working on outreach. Castro Valley Pride was at last year's Fall Festival, and a group marched the past two years in the Rowell Ranch Rodeo Parade, said another of the event's organizers, Austin Bruckner, who graduated from Castro Valley High in 2012.

"Rowell Ranch was a parade of cowboys and us," he said, laughing.

"It was a great," Bradford said. "We were the first ever LGBT entrant into the parade, and we were warmly welcomed."

Both men said they have been gratified by the support they have received throughout most of the community. "Many, many families have attended Castro Valley Pride and said they came to support us," Bradford said. That support is needed for gay young people in Castro Valley, he said.

"Why is there a Pride in Castro Valley? As long as there are churches in Castro Valley that preach against us to their congregations, we need to stand up in our community and say there's nothing wrong with us," Bradford said.

Though the event has grown and is better organized, it's still very much a homegrown affair, he said. There is no corporate sponsor, and the group is not affiliated with any nonprofit agency.

"It's still some high school seniors and some moms and dads," Bradford said. "We're just a group of people in Castro Valley who thought this was important."

Castro Valley Pride
When: Noon-5 p.m. Saturday
Where: Castro Valley High School parking lot, 19400 Santa Maria Ave. (at Redwood Road)
Details: castrovalleypride@gmail.com, www.facebook.com/cvpride