California's vote last week to ban same-sex marriage will hit the state's economy by making it harder for cutting-edge industries like biotechnology to recruit top talent, San Francisco Mayor Gavin Newsom said on Wednesday.

Newsom said the passage of Proposition 8, which amended the state's constitution to halt same-sex marriages less than six months after a state Supreme Court decision legalised them, gives an edge to Massachusetts and Connecticut, which permit same-sex marriage and compete with California in key sectors.

"It's a big economic loss to California, and I don't think people have focused on that enough," he said in an interview.

"Our human capital is what defines our greatest competitive asset," he added.

"It's harder now to recruit people in the gay and lesbian community. It's harder to retain members of the community. It's much easier for Massachusetts to do that."

Newsom was in Shanghai to launch San Francisco's first overseas office, as part of an economic development initiative to boost Chinese investment in his city.

That move as well was framed as motivated by intense competition among U.S. cities and regions, with Newsom noting that Chicago's mayor, Richard Daley, was actively cultivating ties in both Beijing and Shanghai.

"We're seeing a much more competitive environment throughout the United States," he said.

"Where California used to lead in almost every category, and we're still among the most successful, we are no longer the only players. We've got to do more in terms of aggressively competing with these other regions of the country."

Newsom has faced questions that, in the wake of Proposition 8's passage, his high-profile opposition to the measure could have negative implications for any ambitions for statewide office, although he has rejected the idea that his stance on the issue had any political motivation.

He reiterated on Wednesday that he believed last week's setback for supporters of same-sex marriage in California was only temporary.

"I think time is on our side," he said. "I'm absolutely confident we'll get there. But in the interim, economically this has hurt us."