Virginia Ruiz could hear the bull "crying" as she launched herself into the fighting pit in Malaga on Friday and walked to where the animal lay dying.
The 38-year-old put her body over the bull in an attempt to shield and comfort him before she was hauled away by officials and fans.
"Because I was walking, not running, they [the crowd] didn't yet realise I am an activist until I got very close to the bull.
"He was still alive before they could do the final stabbing with the small knife in the back of the neck. He was crying, he tried looking at the people," Ms Ruiz told The Dodo.
As the animal rights activist was pulled off the bull, fans reportedly shouted at her to get out. The bull was later stabbed to death.
Ms Ruiz said she was kicked, spat on, told to go back to the kitchen and called a "whore".
So far this summer, seven people have died after being gored by bulls during festivals across Spain - four of them were killed this weekend.
Despite the unusually high number of people being killed as a result of the bull-running, there has been a growing amount of attention in recent months on the terrible cost to the animals involved in these Spanish events.
The most recent human casualty was matador Miguel Ruiz Perez, who was gored to death by a bull after taking part in an annual summer festival in Lerin, northern Spain.
Ricky Gervais has led the charge against bullfighting and associated events, using his millions of followers on social media to grow awareness and condemn the "barbaric" sport.
The 54-year-old comedian thanked all the "wonderful Spanish people" who want to see an end to bullfighting.
Ms Ruiz, who has been an activist opposing bullfighting since 2008, attended the bullfight alone on a day that had free admission. She said that she was there in order to film the cruelty in the ring.
Ms Ruiz said that she feels "ashamed" that the bloodsport is allowed to continue in Spain and said that she wants the world to know that it does not "represent" the majority of Spanish people.
She added: "There is more to Spain than bullfighting. We are a wonderful people. We are seen as a third world country instead of European."
Last year more than 7,200 bulls and steers (castrated bull calves) were killed by bullfighters across Spain, the news website El Diario reports.
Ms Ruiz stands by her actions, saying she knew she did the right thing. She now faces a notice for trespassing and will have to pay a fine of up to 6,000 Euros.
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