SAN FRANCISCO — The gap widened between the leading candidates in the race for San Francisco mayor, but it is still too close to call.

London Breed, the president of the Board of Supervisors, led former state Sen. Mark Leno by 1,580 votes on Sunday.

The lead has changed hands in recent days. Leno had a margin of fewer than 150 votes on Friday and Breed captured the lead Saturday with nearly 500 more votes.

Election officials say approximately 25,000 ballots need to be counted.

Under the city's unusual ranked-choice voting system, voters select their top-three favorites.

The candidates with the least votes are eliminated in rounds until there's a winner. The person with the most first-place votes isn't necessarily the winner.