The two were wearing t-shirts reading "Stop blessing corridas" and holding banners saying "Bullfighting is a sin".

PETA has been pleading with the Pope to cut the Catholic Church's ties with bullfighting and condemn the "despicable blood sport".

According to the organisation, each year, tens of thousands of bulls are killed in bullfighting festivals globally, many dedicated to Catholic saints. In these events, mounted assailants thrust lances and banderillas into the bull, causing acute pain and restricting its movement.

"As numerous countries are wisely banning this sick form of entertainment, Pope Francis must immediately denounce this blood sport and cut the Catholic Church's shameful ties with bullfighting," it said in a recent statement on its website.

The Vatican didn't immediately comment on Wednesday's protest.

British priest Terry Martin has recently criticised bullfighting in a campaign with PETA and called on Pope Francis to condemn it.

The priest from West Sussex, UK, posed in a red chasuble next to a bull with the inscription: "It is a sin to torture animals".

PETA has pointed out that Pope Francis wrote in his encyclical Laudato Si' that "any act of cruelty to any creature is contrary to human dignity and that, as early as the 16th century, Pope St. Pius V banned bullfights that were deemed cruel and far removed from Christian piety and charity".