Officials said Fr Mark Ventura, 37, was shot in the head and chest by a gunman just after 8am.
The gunman fled with an accomplice on a motorcycle.
Gattaran police chief inspector Rodel Tabulog told CNN the motive of the killer is not known yet but a special investigation task group has been created to investigate.
Cagayan Police provincial director Senior Superintendent Warren Tolito said Ventura was an advocate for free and fair elections and had an interest in Filipino workers' rights, and children's welfare.
Condemning the killing, the Catholic Bishops' Conference of the Philippines said in a statement: "We make our appeal to the authorities to act swiftly in going after the perpetrators of this crime and to bring them to justice.
The opposition Liberal Party also called for justice and in a statement said: "We call on the authorities to capture and prosecute Fr Ventura's killers as soon as possible and not treat Fr Ventura's death as just another death under investigation."
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