"We want to sensitise people to the fact that animals are capable of feeling and suffering like us, and to protest against the massacre of hundreds of thousands of seals which is about to begin in Canada," said spokeswoman Silvia Toval.
Canada is home to the world's largest annual commercial seal hunt.
The country, which defends the 350-year-old hunt, has authorised the culling of 275,000 seals on the Atlantic coast this year, nearly a third of the number killed annually worldwide.
Over 40 percent of the seals killed last year were still alive when they were skinned, according to Toval.
Equanimal said it wanted to pressure the European Parliament, which is due to decide next month whether to impose a complete ban on seal products in the 27-member bloc.
"This is the first time that I get naked for a cause like this," one of the participants in the protest, Jose Antonio Polo, told public television TVE.
"It is very sad because I put myself in the skin of these animals who are treated like objects and they are not objects, they are individuals who have the right to be born and live in freedom," he added.
The seals are hunted mainly for their pelts, but also for meat and fat, which is used in beauty products.
According to the European Commission, Canada, Greenland, and Namibia account for about 60 percent of the 900,000 seals hunted each year, with Canada being the biggest source.
Seals are also hunted in Iceland, Norway, Russia, and the United States as well as in EU member states Britain, Finland and Sweden.
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