If no candidate receives a majority, the top two vote-getters will meet in a Nov. 8 runoff. Recent polls showed that to be a likely scenario.
The winner will enter a City Hall left in shambles after several scandals and problems that have tarnished San Diego's image and threatened it with bankruptcy.
Dick Murphy, the former mayor, announced his resignation soon after his November reelection, saying he wanted to give San Diego a fresh start after his disputed election battle with Frye. His departure came amid a widening federal investigation of the city's pension fund.
Less than 72 hours after Murphy left office, his interim replacement, Councilman Michael Zucchet lost the job when a jury convicted him of corruption charges for taking payments from a strip-club owner in a failed scheme to overturn a ban on patrons touching dancers at nude bars.
Frye has defied expectations since she emerged on the political scene four years ago, inspired to take action by the ocean pollution sickening her husband and friends.
She has asserted that she would reform City Hall by ending a culture of backroom deals that she says have led to San Diego's woes.
Margarita Carmona, a 68-year-old retired school administrator, cast her vote for Frye, saying her honesty and integrity is the antidote for the city's woes.
''I feel embarrassed to say I live in San Diego, with all the corruption that's going on right now," said Carmona, a lifelong resident. ''We need a good, strong leader to pull us out of this mess and to unite us."
Francis, a 50-year-old founder of a hospital-staffing company, was a political unknown in San Diego until he spent about $2 million on a campaign that promoted himself as a businessman and outsider.
''I really feel we have the wind at our backs," Francis said after casting his ballot in early voting Monday. ''We've come from virtually nowhere in the polls to, some would say, dead even . . . with Jerry Sanders."
Sanders, 55, portrayed himself as a turnaround specialist, touting his record as police chief from 1993 to 1999 and positions at the local United Way and American Red Cross chapters.
Councilwoman Toni Atkins is serving as interim mayor until a full-time replacement is elected.
