Visitors to the Serpentine Gallery in London were met by an unusual sight at the weekend: students and graduates dressed as Father Christmas, protesting against a gallery advert for unpaid work.

The group of white-bearded protesters held a banner that read: "All we want for xmas is pay." They handed out scrolls to members of the public, which contained the original advertisement, plus the protesters' comments about why they felt the role should be paid. One said: "If you contribute to the running of an organisation, you are entitled to national minimum wage."

Following the protest, the Serpentine Gallery have admitted it is in the wrong, saying: "The points you make are valid and we have listened to your protest. We take our responsibility as employers very seriously, and this advertisement is not in line with our current terms on volunteer placements."

Woody Morris, a fine art and history of art student at Goldsmiths University and a member of campaign group Future Interns, was one of the Santas at the protest. He says: "It was quite funny because it turned out that Saturday was a thing called SantaCon, so the park was just full of Santas.

"We thought it was amazing, because the gallery wouldn't really think anything of seeing us in the outfits.

"The theme worked well because people were more likely to accept the scrolls because we said Merry Christmas to them. We weren't noisy and causing trouble, we were just talking to people, so I think it was a good way to protest."

Morris spoke to gallery-goers about the cause and the reaction was generally supportive, he says.

Photograph: Sara Gharsalli

"Since we have been campaigning against unpaid internships, people have been sending us adverts that they have seen," says Fairooz Aniqa, another member of Future Interns and an organiser of the protest.

"We thought that this was a particularly bad one that seemed like it should be paid because it outlined the role."

As well as the festive protest, the Precarious Workers Brigade, who also campaign against unpaid internships, sent a letter to the Serpentine Gallery explaining why they felt the voluntary role advertised warranted the national minimum wage.

The group says: "It is clear that the job requires previous experience and some expertise in the area. While, as a charity, it is not a legal necessity that you pay what are described as 'volunteers', many in the sector have acknowledged the exploitative implications of taking advantage of this legal loophole and not paying young workers who clearly fulfill necessary, vital and in many cases, formerly paid positions within organisations."

Because many arts organisations are charitable institutions, they can advertise for volunteer roles, but voluntary work comes with an understanding that the hours are flexible and commitment is genuinely voluntary. The government's guide to internships and volunteering outlines these differences, and the Arts Council's guide to internships offers advice for arts organisations on employing interns.

The gallery initially responded to the Precarious Workers Brigade letter by saying: "All volunteers are reimbursed travel and food expenses, volunteer roles require much less commitment than an internship and are not core roles that we would otherwise have in the organisation.

"Often these roles give people the experience they require as part of their personal development. We often take volunteers without previous experience and skills are presented merely as suggestions to candidates."

But in a later response, sent to the Precarious Workers Brigade and Guardian Students, a spokesperson for the Serpentine Gallery said that the advertised role was "not in line with our current terms on volunteer placements".

Morris says: "I've never done an unpaid internship and I feel very lucky to have a paid job at an arts organisation. But I'm still a student, so when I graduate I might feel like I have to do one to get ahead, and it's really hard.

"I feel that no one should ever be expected to work for free. There are certain times when voluntary work is acceptable, but the thing about this job was we felt it specified set hours and tasks."

Future Interns say they will be continuing to put pressure on art galleries and other institutions who advertise for unpaid roles.