In the four-hour documentary directed by Dan Reed, which aired in a slightly shorter format in the UK, Wade Robson, 36, and James Safechuck, 41, have alleged that Jackson groomed and molested them as children, CNN reported.
Robson met the singer at age five, after winning a dance-alike concert in his native Australia, while Safechuck was eight when he appeared alongside Jackson in a 1986 Pepsi commercial.
The London ads show a black and white photo of Jackson's face. One version has the word "Innocent" across his mouth and reads "Facts don't lie. People do".
The other displays the hashtag "#MJINNOCENT".
Both versions direct viewers to the website mjinnocent.com, which lists purported evidence of the pop-star's innocence.
Anika Kotecha, a lawyer who is one of the organisers of the campaign, told CNN that the posters were the work of a small Britain-based team.
"We've known for a long time that these allegations are nothing but lies and we wanted to get that message out to the general public," she said.
Kotecha and other organisers used an agency to place the ads, although she told CNN that they liaised with government body Transport for London in the process.
The ads will be displayed into April, on a total of 60 buses, the lawyer added.
The MJInnocent campaign previously released T-shirts featuring the same imagery as the bus posters.
Ahead of the UK broadcast of "Leaving Neverland", Jackson defenders protested outside the headquarters of British television network Channel 4, which aired the documentary.
After the film aired at the Sundance festival in January, Jackson's family called it a "public lynching" and have filed a suit against HBO, which co-produced the documentary.