TORONTO - An ambitious young Toronto city councillor with his sights set on the mayor's office abandoned his bid to lead Canada's most populous city Wednesday as he apologized for multiple incidents of sexual infidelity.

"There are weeks that change your life and this one has certainly changed mine," Adam Giambrone, 32, told a packed news conference.

"This searing experience has taught me, I hope permanently, that a public career of integrity cannot survive deceit in your private life."

Giambrone apologized to his supporters, friends and family for the hurt he has caused after admitting to having "intimate relations" with a number of partners over the last year in the run-up to the election, scheduled for October.

Giambrone was too distraught to finish his prepared statement, instead fleeing right after apologizing for the "deep hurt" he has caused to Sarah McQuarrie, his partner of several years.

McQuarrie was at Giambrone's side when he announced his candidacy Feb. 1 Giambrone reportedly said in an email to one of the women that McQuarrie is around for "political" purposes.

"I deeply apologize to my partner Sarah," said an emotional Giambrone. "The pain she has endured for my mistakes was deeply unfair to her. She has shown a lot of strength in all of this."

Giambrone then quickly exited as reporters shouted questions after him. His assistant, Kevin Beaulieu, had to read the rest of Giambrone's prepared statement for him.

"His mayoralty campaign ends today," Beaulieu said.

Giambrone, who was first elected to council in 2003 at age 26, is an archeologist by training and is fluent in French and Arabic, according to his website. He also served as the national president of the New Democratic Party of Canada from 2001 to 2006.

Giambrone will stay on as a city councillor and chairman of the Toronto Transit Commission, the country's largest, yet much maligned, public transit system, despite several other councillors calling for his resignation from that agency.

This week a 20-year-old university student came forward saying she had been having an affair with Giambrone. The councillor at first said the relationship was limited to texting and conversations in public places. He then admitted to having "intimate relations" with women other than his partner.

The student has said Giambrone discussed a fair hike at the transit commission before it happened.

The relationship and Giambrone's further admission of multiple affairs came to light in the Toronto Star.

Giambrone had no choice but to resign, according to one public relations expert who has worked for many high-profile politicians.

"In short, it's a nightmare. This is one of those situations where all the spin-doctoring in the world just can't save you," said Bob Reid, chief media strategist for Veritas Communications.

"When you have a candidate like this who has absolutely shredded his credibility, and I think he did that right on day one, all the counterspin, all the counter positioning in the world just isn't going to pull it out of the fire," added Reid, who is not working for, or supporting, any of the mayoral candidates.

Mayor David Miller called Giambrone's decision regrettable but understandable.

Mayoral candidate George Smitherman called on him to resign as TTC chair.

Coun. Howard Moscoe said Giambrone probably did the right thing by stepping down. When asked what would happen if every politician who was having an affair resigned, Moscoe predicted: "We'd have a very small council."

A number of other city councillors expressed the view that a politician's private life should remain private.

But it's fair game to report on the sexual affairs of politicians if they lie to the public, says Christopher Waddell, the director of Carleton University's School of Journalism.

"When politicians create images for themselves, and if they do things in their private lives that are contrary to the public image they're creating for themselves, then I think they become fair game for media reporting," he said.

Waddell says the truth will come out in a world of emails, Facebook, Twitter, websites and digitized media.

Candidates on the mayoralty ballot now include Smitherman, former Liberal fundraiser Rocco Rossi and two city councillors, Joe Pantalone and Giorgio Mammolitti.

Before he declared his candidacy, Giambrone posted a tongue-in-cheek video on YouTube in January showing him doing push-ups, jumping jacks and jogging in place to illustrate his fitness for the job. He also practised hand-shaking and speech-making in front of a mirror, saying that it takes "years of physical and mental training to prove you can run a city."

Giambrone's position with the transit commission is not without its challenges.

Relations between TTC workers and passengers have soured in recent weeks and descended into a war of words through the media and social networking sites.

A photo showing an employee asleep in his subway ticket booth was widely posted online and a video then surfaced of a bus driver allegedly taking an unscheduled coffee break on his late-night route.

TTC general manager Gary Webster issued a stern warning to employees that he also made public, which angered some workers who threatened a work-to-rule campaign that ultimately never materialized.