The curfew, called on Thursday, comes as Iraq's Association Muslim Scholars called for an investigation into the Ashura day attack by US and Iraqi forces on a group just outside Najaf.
Ahmad Duaibel, a spokesman for the provincial authority, added: "Authorities took this decision after receiving information that armed groups planned to launch attacks against the city."
Güncelleme Tarihi: 20 Eylül 2018, 18:16
Police in Najaf said: "The curfew imposed overnight has been extended and includes the entire region around Najaf including Kufa, northeast of the holy city ... Police checkpoints have been told to forbid people from entering or leaving the city, and this order applies to residents as well."
More than 250 people were killed and almost 300 arrested in a battle on Sunday between an alleged doomsday cult and US and Iraqi forces.
The authorities say a man known as Ali bin Ali bin Abi Talib, the alleged leader of the cult, was killed in the attack.
At least six members of the Iraqi security forces died in the fighting and two US soldiers were also killed when their helicopter crashed during the fighting.
The fighting ended early on Monday, a defence ministry spokesman said.
Officials had said earlier this week that the "cult leader" had planned attacks on senior Shia clerics in Najaf and the group was preparing to occupy Najaf.
But on Friday, the Association Muslim Scholars called for an investigation into the incident, which they termed the "al-Zarka massacre".
The association said it did not have confidence in the government over the matter and called the incident nothing more than the targeting of Arab clans who were not supporting the government.
The association said the government's claims about the cult called "Jund al-Samaa", or the "Soldiers of Heaven", were baseless.