A terrorist plot to attack the Australian city of Melbourne with explosives, knives and guns — possibly on Christmas Day — is believed to have been headed off with the arrest of five people, Prime Minister Malcolm Turnbull said on Friday. Mr. Turnbull told reporters in Sydney the group was “self-radicalized” and inspired by the Islamic State. Five men, ages 21 to 26, were to be formally charged later Friday, the authorities said. Another man and a woman were also taken into custody, but were later released. According to the police, the plotters had scouted Federation Square, a popular shopping and entertainment district, as well as the Flinders Street train station, Melbourne’s busiest, and St. Paul’s Cathedral, an Anglican church. All three locations are close to one another in the center of the city.