OAKLAND -- In a move that would have seemed unthinkable three years ago, Mayor Jean Quan's former legal adviser and longtime friend has decided to challenge her in this year's mayoral race.

Civil rights attorney Dan Siegel has known Quan since their days as student activists at UC Berkeley nearly a half-century ago. He served as her unpaid legal adviser after she was elected mayor in 2010, but they became estranged over Quan's decision in 2011 to evict the Occupy Oakland encampments from in front of City Hall.

Now Siegel is positioned to carry the mantle of Oakland's political left against Quan, who has moved to the center during her term in office, which has been dominated by concerns about crime and diminished police staffing.

"I think that Mayor Quan has lost the initiative that she seemed to be taking at the time of her election and has not pressed for the kinds of polices that I think we need," Siegel said. "I'm just going to be providing a sharp alternative to the other candidates in the race."

Quan's campaign co-chairman, Michael Colbruno, said the mayor welcomed the competition.

"The mayor is going to run on her record, and we think it's a winning record to run on," he said.

Siegel served two terms on the Oakland school board and briefly ran for mayor in 2006 before dropping out to support Ron Dellums. He is considered the most left-leaning challenger to Quan in a field that includes Oakland City Councilwoman Libby Schaaf, San Francisco State Professor Joe Tuman, Oakland Port Commissioner Bryan Parker and North Oakland resident Patrick McCullough.

"Now the entire political spectrum is probably covered because we go from the far left to the far right," former Councilwoman Marge Gibson Haskell said.

It's difficult to gauge whether Siegel's candidacy will hinder Quan's re-election hopes because voters in Oakland are asked to rank their top three candidates. That means even if Siegel takes away some first-place votes from Quan, she could be well-positioned to be the second or third pick of Siegel's voters, which still could help her cause.

A recent poll commissioned by the Jobs and Housing Coalition showed Quan garnering the most first-place votes among declared candidates but struggling to pick up critical second-place votes.

Siegel said he would campaign for a $15 citywide minimum wage, enhanced preschool programs, and police reforms to put more officers on patrol and solve more murders.

He also said he would abandon the city's soon-to-be completed $10.9 million surveillance hub, which detractors say would give authorities new tools to monitor residents.

"I'm astounded that members of the City Council and the mayor's office seemed to have ignored the national outrage over spying by the NSA and seem willing to install a municipal NSA program here in Oakland," Siegel said.

Siegel said he will withdraw from his law practice, if elected. He represents the family of Alan Blueford, who was fatally shot by an Oakland police officer on May 6, 2012, as well as about 400 people who were arrested during an Occupy Oakland protest on Jan. 28, 2012.

Siegel said he plans on running a positive campaign. He also said he holds no animosity toward Quan but acknowledged their relationship has not recovered since he withdrew as her legal adviser and criticized her after the second Occupy camp eviction.

"Things are very cool between us," he said. "But on a personal level, I still view Jean and members of her family as friends. We've just come to a point of political disagreement, which happens among friends."

Contact Matthew Artz at 510-208-6435