JERUSALEM, Oct 2 (Reuters) - Suspected hardline Israelis scrawled pro-settler graffiti and religious insults on a monastery outside the walls of Jerusalem's Old City on Tuesday, police said, in the latest of a series of attacks on non-Jewish sites.

The vandals wrote the phrase "price tag" in Hebrew on the gate of the Monastery of Saint Francis on Mount Zion - a reference to a violent campaign supporting unauthorised settlements in the Israeli-occupied West Bank.

The "Price tag" is the retribution some Israeli settlers say they will exact for any attempt by their government to curb settlement in the territory, which Palestinians want as part of a future state.

The group has targeted mosques and, less commonly, Christian buildings, regarding any non-Jewish religious sites as an intrusion on the land.

The monastery of Saint Francis is near the spot where tradition says Jesus gathered his disciples for the Last Supper.

It was the second attack on a Christian institution in less than a month.

"Price tag" attackers set fire to the doors of Latrun monastery in the West Bank on Sept. 4 in a possible retaliation for the eviction of families from an unauthorised outpost.

"Price tag actions are contrary to the Jewish religion and causes great harm to Israel," Israeli President Shimon Peres said in a statement in response to Mount Zion attack. "Holy sites must not be harmed."

A police spokesman said a number of people had been charged in connection with several of the incidents, but gave no details.

Palestinian officials and Israeli rights groups have accused the authorities of not doing enough to investigate the attacks. (Writing by Ori Lewis; Editing by Andrew Heavens)