OAKLAND — Oakland will move to an instant-runoff voting system this year after the City Council approved the switch 6-2 late Tuesday.

The change precludes the need for a June primary in the city's mayoral and council elections, and the decision followed an opinion from City Attorney John Russo that said officials had no choice but to follow the mandate of 2006's Measure O and implement the system.

Even so, the council's vote appeared in doubt heading into the meeting, as some expressed concerns about the city's budget problems and whether voters would be properly educated about the new system.

Measure O was approved by 69 percent of Oakland voters in 2006, and dozens of supporters and detractors turned out to voice their opinions on whether it was the right time to make a change.

"The people spoke," Judy Cox, president of the Metropolitan Greater Oakland Democratic Club and one of the city's most persistent advocates for instant-runoff voting, told council members. "Sixty-nine percent of them wanted this. That is a mandate in my book."

Supporters hailed the council's vote as a victory for stronger democracy in Oakland, saying November elections, as opposed to June primaries, draw more voters to the polls. Opponents said the timing was wrong to launch a new voter system and to educate residents about it when Oakland is facing multimillion-dollar deficits.

In instant-runoff voting, also called ranked-choice voting, voters rank candidates in order of preference, eliminating the need for separate runoff elections when one candidate does not reach a 50 percent threshold.

There will be an upfront cost: The county says it will charge as much as $1.5 million to pay for voter education and equipment upgrades to launch instant-runoff voting in Oakland, Berkeley and San Leandro. Oakland's share could be as much as $946,000, but about $800,000 of that could be saved if there is no election in June — an election is still a possibility because some have discussed placing a public-safety ballot measure before voters this spring.

The move to instant-runoff voting carries implications in Oakland's mayoral contest. Most observers thought candidate Don Perata, who lobbied against instant-runoff voting, would have gained an edge if a June election were held because of his fundraising abilities and name recognition, whereas Councilmember Jean Quan (Montclair-Laurel), also a mayoral candidate, could benefit from a November-only election season.

Larry Tramutola, a senior adviser to the Perata campaign, said earlier Tuesday that he did not think the council vote would have any effect on the election. Perata opposed the move because he considered the voting system questionable and because of the city's strapped resources, Tramutola said.

Councilmember Ignacio De La Fuente (Glenview-Fruitvale), a Perata ally, launched a bid Tuesday to derail the change, saying it was too costly.

"I think the same people who are here tonight supporting this ranked-choice voting will be here in the next several months asking us not to cut and not to reduce programs," he said.

City Council President Jane Brunner (North Oakland) also voted no, saying she wanted more specifics in the city's agreement with the county about how the plan will work. The other six council members supported the change.

"Many Oakland voters have been waiting for this a long time," said Patricia Kernighan (Grand Lake-Chinatown). Kernighan, along with Councilmember Nancy Nadel (Downtown-West Oakland), was one of the original supporters of Measure O, which called for Oakland to move to the new voting system once it was possible to do so.

California Secretary of State Debra Bowen approved Alameda County's plans for the new voting system in December, and Russo followed by saying that under Measure O, the council legally did not have the option of delaying the change.