Six half-naked animal rights demonstrators began a string of protests planned in the days before Pamplona's traditional "running of the bulls" begin next week.

The group reportedly interrupted Madrid's historic Puerta del Sol square overnight in a protest against bullfighting. Shoppers and tourists snapped photos as the five young women and one man carried "Stop the Bloody Bullfights" signs held by the protesters, wearing only white underpants and plastic bulls horns on their heads. The chanted, "Torture is not art or culture".

Leonora Esquivel, president of animal rights group Anima Naturalis, estimates some 70,000 bulls a year were killed in bullfights in Spain. She calls it the worst animal rights record in Europe.

Pamplona's ancient San Fermin festival begins next Wednesday and draws tens of thousands of visitors during a week-long running bull fiesta. The bull runs along an 825-meter course to the bullrings that take place each morning.

The festival gained popularity after Ernest Hemingway's 1920s novel The Sun Also Rises.