As the men of Shetland gathered in Lerwick early on Tuesday morning for the dramatic commencement of Up Helly Aa, Europe’s largest fire festival, they were greeted by an unexpected sight.

A banner depicting a female Viking warrior astride her horse hung down the stone pillar at the Market Cross in the town centre, with a direct challenge to the organisers of the exclusively male procession: “Have you forgotten those that bore you? Act lest the gods should intervene!”

The Viking festival attracts visitors from across the globe on the last Tuesday in January each year, as near a thousand heavily-costumed men, in groups known as “squads”, parade through the streets of Shetland’s capital, culminating with the burning of a replica long ship.

But the organising committee has faced increasing criticism in recent years for its refusal to countenance the admission of women and girls to the squads, or to recognise the role of female warriors in Viking folklore, while female volunteers are restricted to taking part as “hostesses”, organising the all-night parties that follow the torchlight festivities.

While the question of women’s involvement has divided opinion among islanders, Tuesday was the first time that equalities campaigners have taken such direct action on the day of Up Helly Aa itself.

In a statement posted on social media, the Reclaim the Raven group, which was responsible for the protest, warned: “No one wishes to see Up Helly Aa become better known for the exclusion of woman and girls than for being the biggest and best fire festival in Europe.”

Referring to Frigg, the goddess of foresight and wisdom, the group added: “We would hope the Up Helly Aa committee would show these and engage with everyone in bringing a bright and inclusive future to this festival that is loved around the globe.”

The smaller South Mainland Up Helly Aa appointed its first female leader five years ago but the main festival remains all-male.

Last May, the campaign group Up Helly Aa for Aa complained to Shetland Islands council after an application by four schoolgirls to join the junior parade was rejected by organisers, but the council ruled the decision was in line with the Equality Act 2010. Then in August, four women reported they had been blocked from registering a mixed gender squad called #MeToo for this year’s festival.

Traditionally, Up Helly Aa day kicks off when the bill of the “Guizer Jarl”, the annually chosen leader of the festival, which sets out the squads’ duties and mocks local events of the last 12 months, is erected at the Market Cross. The “alternative” bill from Reclaim the Raven was set up some time around 5am, but removed by committee members later in the morning.

Zara Pennington, a member of the group who was present at the early morning unveiling of the proclamation, said: “There is definitely a growing public mood in favour of discussing this. The reactions to the protest so far today have been mainly positive, and there’s a sense that the community is gaining its voice, whether it’s those in favour of letting women join, or those who haven’t considered the issue before, rather than continuing with the accepted narrative that this is tradition.”

Pennington explained: “Myself and many others do find it embarrassing to see the reaction of our many international visitors, when they realise that this is a men-only parade. With the Hawick Common Riding [the Borders horse-riding festival] now letting women participate, this is the only major event in Scotland that keeps women apart.”

The Up Helly Aa committee has been approached for comment.