The giant Muni platform near Chase Center — big enough to hold 700 fans or a small-town parade — opened for business Tuesday.

It’s a key piece of San Francisco’s strategy to lure sports crowds out of cars and onto mass transit. And the Warriors are emphatically promoting Muni, offering a free day-long bus and train pass to anyone who buys a ticket to a basketball game, concert, or other event in the 18,000-seat arena set to open this month. The team also paid for four new Muni light-rail cars to beef up the T-Third line.

Mayor London Breed, Assemblyman David Chiu and Warriors President Rick Welts cut the platform ribbon shortly before noon, as riders on a passing T-Third train craned their necks to watch. Workers in orange vests and hard hats paced the trackway, removing orange traffic cones and clearing debris. The platform bore new signs and a freshly scrubbed awning. Behind it stood Chase Center, a futuristic swirl of glass and metal.

Minutes later, an empty Muni train made a ceremonial stop, its doors sliding open so a throng of politicians and reporters could step inside. The platform opened for passengers at about 12:15 p.m.

“This neighborhood, which basically used to be a railway and empty space, has now become a real part of San Francisco,” Breed said, gesturing to the busy intersection at Third and South Streets, in the heart of Mission Bay. The once-fallow area now has restaurants with patio seating, bike rental docks and a hospital.

Now, with sports crowds expected to barrel in by the thousands, the redesigned Muni stop will serve as a vital piece of infrastructure, Breed and other officials said. Combined with new protected two-way bike lanes and a temporary ferry landing near Mission Rock, it will help ease the crush of traffic on city streets.

“The Warriors arena will be the most sustainable, the most transit-friendly, the most accessible arena in the NBA,” said San Francisco’s interim transportation chief Tom Maguire, speaking at the ribbon-cutting.

The platform, which took nearly a year to build, will allow T trains to pick up and drop off passengers more efficiently, providing enough space for four two-car trains to load during events. San Francisco also is promoting myriad other forms of mass transit for those attending Chase Center events including valet bicycle and scooter parking, shuttles from downtown BART stations and ferry service from Larkspur, Oakland and Alameda.

Eventually, transportation officials hope to open a permanent Mission Bay ferry terminal — a dream that’s captivated Chiu, D-San Francisco.

“This is not going to be the last transit-stop ribbon cutting,” Chiu said, adding that he hopes to unveil the new Mission Bay dock in 2021.

Drivers may experience difficulty. The center garage holds only about 900 parking spaces, in addition to about 2,000 in the vicinity, all priced at $7 an hour during special events. The city will bar most private cars — including ride-hail vehicles — from several blocks surrounding Chase Center.

Rachel Swan is a San Francisco Chronicle staff writer. Email: rswan@sfchronicle.com Twitter: @rachelswan