Members of Fathers 4 Justice (F4J) staged a naked protest in the flagship London branch of Marks & Spencer at Marble Arch, Oxford Street.
Group founder Matt O'Connor was arrested during the weekend protest, which criticised the high street store for its advertising on parenting blog site Mumsnet.
Along with four other members of the group, O'Connor took off his clothes in the store to draw attention to what F4J claimed was Mumsnet's "anti-male agenda".
He claimed that the protest was just the start of a series of attacks on M&S in order to draw attention to the "naked truth" that Mumsnet "promotes gender hatred".
He told IB Times UK: "We discovered some horrific content on the Mumsnet forums, including calling men fascists and equating fathers with Ku Klux Klan members.
"Of course, these comments were on open forums, but some of the language used about us was just horrific and we thought that something had to be done about it.
"We complained to Mumsnet and, frankly, expected them to start investigating it straight away. It is a popular and well-used website and we assumed it would want nothing to do with this content."
Dissatisfied with the response from Mumsnet, the group decided to target Marks & Spencer, which regularly advertises on the site.
"We decided to follow the money and draw attention to the company's connection with a website that had such hateful content on it. This is very serious issue, but they just failed to engage with us."
O'Connor said the group, which regularly drew media attention for its members dressing up like superheroes, was keen to tone down its protest action and concentrate on political discourse, but made an exception in this case.
The group launched its Say Pants 2 M&S protest, calling for a boycott of the company. O'Connor and three other group members entered an M&S in Marble Arch and stripped to their pants before eventually going naked.
Police were called and members of the group were arrested for outraging public decency.
O'Connor said: "We wanted to expose a naked truth, so why not be naked. There is a form of gender apartheid that is happening when it comes to the treatment of men.
"When you look at the language being used in some of these forums, you can see how unacceptable it would be if it was aimed towards other races or sexualities, but it seems to be widely accepted against men.
"We believe that a child has a right to a father and that all people, no matter the sex, colour or creed, should be treated equally.
"I was arrested, but I know we didn't upset people. There was a very jovial atmosphere and everyone was laughing. Let's be honest I am in my 40s and not exactly in my prime. The whole experience was quite liberating though."
He said the group had informed M&S that the protest was just the start of a "guerrilla" campaign that "will involve male nudity".
He said: "We think the naked side of things is in keeping with the style of F4J. This is a hugely serious issue, but we are happy to be the butt of the jokes as long as it raises awareness of the problems fathers face."
In response to F4J's allegations, Mumsnet founder Justine Roberts wrote an article on the site in which she admitted that "some Mumsnetters said some pretty harsh things", claiming that at least 70 posts were deleted, while users were regularly reminded to obey forum guidelines.
"At the same time, comments on the F4J Facebook page describing Mumsnetters as "barking mad harridans", "weird sex-obsessed paranoid perverts" and "child abusing contact blockers" were left unmoderated," she added.
"The suggestion that Mumsnet encourages gender hatred would be funny if it were not so offensive - and plain silly. The central aim of Mumsnet is to make parents' (mothers and fathers) lives easier".
"We do not pre-moderate or vet comments made on our discussion boards, of which there are around 30,000 every day."
She claims that men have always been welcome on the site, though there is a specific Dadsnet forum.
"Of course, you can always find plenty of Mumsnetters whingeing about their male partners' shortcomings - more than they are whinging about their female partners' shortcomings - but generalisations are swiftly pounced on and we do not tolerate gender hatred, or any other kind of that hatred for that matter. We encourage people to be civil and supportive and, in the main, most people are."
A spokesperson for Marks & Spencer said: "This is a matter between Fathers 4 Justice and Mumsnet," adding that the store was confused why had it had been targeted.
"We have no plans to change our marketing strategy. We are absolutely apolitical and would not discriminate against anyone."