SAN FRANCISCO — San Francisco has become the first city in the nation to outlaw chewing tobacco from its playing fields, including AT&T Park, home to the San Francisco Giants.

Players and the manager of the team expressed support for the ordinance signed into law by Mayor Ed Lee on Friday but also concern about breaking the chewing habit.

The ordinance, which will take effect Jan. 1, prohibits the use of smokeless tobacco at athletic venues, specifically singling out baseball, which has a long history of players masticating and spitting tobacco juice in view of children who worship them.

“It’s a step in the right direction,” said Giants manager Bruce Bochy, who has chewed tobacco on and off for decades.

“It’s a tough deal for some of these players who have grown up playing with it and there are so many triggers in the game,” Bochy added. “I certainly don’t endorse it.”

The San Francisco ordinance is part of an overall push by the Campaign for Tobacco-Free Kids, based in Washington, D.C., which targeted the city and California to promote its anti-smoking efforts. An even more expansive bill outlawing all tobacco use, including electronic cigarettes and smokeless tobacco, wherever an organized game of baseball is played in California is making its way through the Assembly.