The beloved 85-year-old priest murdered by jihadists in his church last week was mourned by thousands of people at his funeral Tuesday amid tight security.

Mourners filled Rouen Cathedral to bid farewell to the Rev. Jacques Hamel, who was knifed to death at the altar of his church in the Normandy town of Saint-Etienne-du-Rouvray as he celebrated Mass on July 26.

Interior Minister Bernard Cazeneuve attended the service, during which Hamel’s coffin was carried through the “Door of Mercy” and placed on an ornate rug before the altar, Reuters reported.

The priest’s sister, Roselyne Hamel, described how during his military service in Algeria, her brother had refused to become an officer so he would not be forced to give an order to kill.

“He would often ask himself: ‘Why me?’ Today, Jacques, our brother, your brother, you have your answer: Our God of love and mercy chose you to be at the service of others,” she told the congregation.

Hamel’s murder at the hands of 19-year-olds Abdel Malik Petitjean and Adel Kermiche was the first Islamist attack on a church in Western Europe and came just 12 days after a Tunisian who had pledged allegiance to ISIS drove a truck through a crowd of Bastille Day revelers in Nice, killing 84.

“As brutal and unfair and horrible as Jacques’ death was, we have to look deep into our hearts to find the light,” said Rouen Archbishop Dominique Lebrun, Agence France-Presse reported.

A red stole — symbolizing Christ’s martyrdom — was placed over a giant cross beside the altar, with the Rouen diocese explaining that “Father Hamel’s death was similar to that of Christ, unjustly convicted and put to death.”

In a show of interfaith solidarity, Muslims and Jews were among the mourners.

“It was a duty,” Hassan Houays, a Muslim math teacher from Saint-Etienne, told AFP. “We are here so that we can get along together.”

Hamel was expected to be buried at an unspecified location in a ceremony attended only by close family members.

Petitjean and Kermiche, who were seen on video as they pledged allegiance to ISIS, were shot dead by police. One of their hostages was left seriously wounded and three other hostages escaped unharmed.

With Post wires

Watch: Video shows cops rushing nun to safety in ISIS church attack