A leading voice in the No campaign has been threatened with defamation action unless it apologises to a public servant whose image was shown beside a headline labelling her and fellow same-sex marriage supporters "extremists".

Canberra woman Jill Moran said she felt "sick to my stomach" when her image, which she said was taken at a peaceful protest, was used in an article on the Coalition for Marriage's website.

The headline on the article, dated September 16, read "tonight a test for the extremists of the Yes campaign", referring to the Coalition for Marriage's official campaign launch.

The article itself included quotes from Sophie York, who spoke broadly about alleged bullying and intimidation within the Yes camp's "base".

Ms Moran said by placing the image next to the headline and article, the Coalition for Marriage had insinuated violent and bullying behaviour on her part.

"It is completely untrue and hurtful to paint me as violent or a political extremist," she said.

Jill Moran says she felt sick to her stomach when she saw the article. ( ABC News: Aarti Betigeri )

"I am thinking of the thousands of people around Australia who have attended rallies in support of same-sex marriage.

"How many of them do you consider an extremist just because you are holding a rainbow flag?"

Ms Moran's lawyer, Phil Johnston, who is representing her pro bono on behalf of law firm Slater and Gordon, said the image had been taken at a peaceful rally and Ms Moran's behaviour had been misrepresented.

"The allegations of violent protesting, of bullying and intimidation with regards to Jill are just patently false," he said.

"She has no history of violence, she has no criminal record, she's just a public servant in Canberra.

"It's damaged her personal reputation — when she called me she was mortified."

Mr Johnston said in a letter sent on September 25 that he had given the No campaign 28 days to remove the photograph from the website and issue an apology and correction.

"Right now all we're looking for is the removal of Jill's photo and an apology," he said.

"If we get 28 days down the track and they haven't come back with an acceptable offer of compromise, then we'll be forced to consider issuing proceedings."

Activists 'not identified, not defamed'

However, a spokeswoman for the Coalition for Marriage denied the organisation had defamed anybody.

"The Coalition for Marriage is fighting for freedom of speech. That includes freedom of political communication," she said.

"An image of Yes activists deliberately and brazenly disrupting a peaceful anti-Safe Schools event in Canberra was shared with our media release.

"There appear many activists in the image and none of them were known to or named by the Coalition for Marriage."

The spokeswoman said Ms Moran, a Greens member, used the image in question as her Facebook profile picture, and had posted it with the caption "smashing the heteropatriarchy".

However, Mr Johnston said the Coalition for Marriage did not have to name Ms Moran to defame her.

"In normal circumstances defamation actions arise from when someone is directly identified," he said.

"However, it's been long recognised by judges that you can be indirectly identified, you don't need to be directly named, and this is a circumstance where that has happened."