A San Francisco cyclist who fatally struck a 71-year-old man in a Castro district crosswalk almost eight years ago has started a social media attack campaign against George Gascón, the man who prosecuted him for felony manslaughter and is now running for the district attorney in Los Angeles.

Chris Bucchere set up the Facebook page Liberals Against Gascón last month, and records available under the social media company’s new disclosure policy show the page has spent more than $5,000 to run anti-Gascón ads on the site.

Under state law, however, political ads of $1,000 or more in a year qualify as independent expenditures, and anyone buying the ads must file reports with the state’s Fair Political Practices Commission.

Bucchere, 43, said he is faxing the necessary paperwork to the elections office in Los Angeles after speaking with The Chronicle.

“If I missed any FPPC requirement, it’s an honest mistake and I’ll gladly fix it and pay the penalties,” he said in an email. “This is my first time running political attack ads, so I’m doing my best and learning as I go.”

As he sorts out the paperwork, Bucchere remains undaunted in his effort to derail Gascón’s hopes in Los Angeles. He’s previously accused Gascón of committing prosecutor misconduct, even after pleading guilty and being sentenced to probation and community service.

“Obviously I wouldn’t be doing this if it wasn’t for my case,” he said. “There’s no reason he should have charged me the way he did.”

Bucchere, who lives in Marin County and works as a software developer, became the first cyclist in the country to be convicted of felony manslaughter after pleading guilty in a case that grabbed national headlines and created a mini media frenzy.

It happened on March 29, 2012, when prosecutors said Bucchere blew through several stop signs and barreled into the intersection at Castro and Market streets, where he struck Sutchi Hui who was crossing the street with his wife.

Hours after the wreck and nursing his own bumps and bruises, Bucchere wrote a now-infamous blog post saying he was “way too committed to stop” at the intersection, and dedicated the post to his “late helmet (that) died in heroic fashion.”

At the time Hui was still alive and Bucchere said he expected him to recover, but when the victim later died of a head injury, the post followed Bucchere and came off as stunningly insensitive.

“This is a man who showed more remorse towards his helmet than he did the family whose lives he turned upside down, and now he’s peddling illegal ads targeting the man who held him accountable,” Gascón’s campaign manager, Max Szabo, said Thursday. “Out-of-touch tech bros everywhere have a new poster child.”

Bucchere said he believes Gascón, who left his San Francisco post in October to run in Los Angeles, was trying to make an example out of him rather than seek justice. He said he took the plea deal to avoid jail time and spare his family and the victim’s family further anguish.

He’s since written a book about his experience, “Bikelash: How San Francisco Created America’s First Bicycle Felon,” and released a series of podcasts and blogs.

He said he doesn’t like incumbent Jackie Lacey in the race either and is choosing to support a third candidate, former public defender Rachel Rossi.

Evan Sernoffsky is a San Francisco Chronicle staff writer. Email: esernoffsky@sfchronicle.com Twitter: @EvanSernoffsky