Jill Moran claims the use of her photograph in a media release denouncing "the extremists of the 'yes' campaign" falsely portrays her as a bully, a violent protester and a political extremist.

Left-wing law firm Slater and Gordon, which is representing Ms Moran pro bono, sent the Coalition for Marriage a cease and desist letter and now a letter of demand threatening a defamation suit.

But the Coalition for Marriage is standing firm, arguing Ms Moran is a seasoned political activist and that it is fighting for "freedom of political communication".

Slater and Gordon defamation lawyer Phil Johnston said Ms Moran was "pretty distressed" when she contacted the firm, and rated her legal position as strong.

"The accusations are pretty strong - we're talking about bullying and intimidating, violent protest," he said. "By putting her picture up there, she has been associated with this kind of conduct."

Ms Moran, 26, works for a Commonwealth department and is a member of the Greens. She said she tried contacting the Coalition for Marriage directly before engaging lawyers, but did not hear back.

"When I first saw it I thought: what is this? I was shocked," she said. "It's so obviously me in the photo that anyone can look at it and go 'that's Jill'."

The photograph was taken at a demonstration in Canberra organised to counter a rally against the Safe Schools program. According to a Canberra Times report of the event, "tempers flared on several occasions" but there were no arrests made and the event ended peacefully.

Ms Moran said she did not know who owned the photograph and the version she uploaded to Facebook as her profile picture was sent to her by a friend.

In a statement, the Coalition for Marriage denied it had defamed Ms Moran, accusing her of "deliberately and brazenly disrupting a peaceful anti-Safe Schools event".

It argued it had not named Ms Moran, and noted she had tweeted the image with the caption "smashing the heteropatriarchy". She also made social media posts defending "good friends" who had been arrested at protests.

Ms Moran did not deny those claims, but said her friends were arrested at environmental protests, not rallies supporting same-sex marriage or Safe Schools.

The offending media release was posted to the Coalition for Marriage website on September 16 ahead of the official "no" campaign launch in Sydney that evening.

It quoted spokeswoman Sophie York warning of the "bullying and intimidation tactics" by "yes" campaigners, and said they were too numerous to be considered a fringe group.

She went on to describe the Coalition for Marriage campaign launch as a "test" of whether the "yes" campaign could keep its "extremists" in check. Some on the "yes" side viewed this as an attempt to goad supporters of marriage equality into protesting the event. In the end, it was not disrupted by any protesters.

The portrayal of some "yes" campaigners as militant and thuggish was aided the following week when Hobart anarchist Astro Labe headbutted former prime minister Tony Abbott while wearing a "yes" badge. Mr Labe later said it had nothing to do with Mr Abbott's stance on the issue of same-sex marriage.

Slater and Gordon said the Coalition for Marriage had acknowledged the letter of demand and confirmed it was seeking legal advice. Mr Johnston and Ms Moran said they would be satisfied with a retraction of the offending photograph and a published apology.

"This could end today if the Coalition [for Marriage] was inclined," Mr Johnston said.