Virginia Ruiz, a 38-year-old animal rights activist, jumped into the "La Malagueta" ring in the southern city of Malaga on Friday in front of thousands of onlookers and crouched over an injured bull as it lay on the ground.
She was eventually pulled away as spectators waved white kerchiefs and jeered in protest at her action, a video released by a local anti-bullfighting group showed.
"Virginia showed a great deal of bravery and kindness when she jumped into the ring," the director of Peta UK, Mimi Bekhechi, said in statement released in Spain.
"Hopefully her compassionate act is giving citizens a new look at the archaic, shameful spectacle that is bullfighting," she added in a statement which called on the Spanish government to end bullfighting.
Ruiz told private television station Telecinco that her intention had originally been to film the cruelty in the arena but she could not resist jumping in the ring after hearing the bull moan.
"I wanted to give him love before he left this world," she said.
She faces a fine of 6,000 euros (6,600 dollars) for trespassing.
During Spain's 1939-75 right-wing dictatorship of General Francisco Franco, bullfighting was promoted as a unifying national spectacle.
But its mass appeal has faded with polls showing a growing lack of interest in bullfighting throughout Spain, especially among the young, although it retains a passionate following and leading matadors are treated as celebrities.
Bullfighting has been banned in Spain's northeastern region of Catalonia, which has a distinctive language and culture, since 2012.
Prior to Catalonia, bullfighting was outlawed in 1991 in Spain's Canary Islands, but the fights were never popular there.
Seen as an art form rather than a sport by fans, bullfighting is also popular in southern France and some Latin American countries.