NaBai's deputy and town councilor Uxe Barkos launched the traditional "chupinazo" firecracker from the main balcony of the city hall in Iruņa/Pamplona marking the start of San Fermin.
Judge Gianluigi Marrone, who is a member of the court system of the independent Vatican city-state, said the man was wearing clerical garb and carried documents alleging that he was a priest.
Bio-engineering students Laurent Bonomo and Gabriel Ferez were repeatedly stabbed in Bonomo's south London apartment in what police said were the most "frenzied, brutal and horrific" murders they had seen.
Police said the 41-year-old man shoved aside two Madame Tussauds museum employees, one of whom was assigned to protect the exhibit, and slightly injured one of them.
San Fermin kicks off on Sunday and its program includes a total of 484 events. Compared to previous years there will be more international concerts this year.
Laurent Bonomo and his friend Gabriel Ferez, both 23, were bound and stabbed to death at their apartment, near where the arrest was made. Bonomo was stabbed nearly 200 times while Ferez received nearly 50 stab wounds.
