The measure comes after two days of bloody reprisal in which more than 130 people have died, most of them Sunnis.
Government officials held an emergency meeting to discuss the crisis, and those involved said talks included concern of the risk of civil war and that tensions could spill over during Friday prayers.
An overnight curfew has been in place for the past two years, and has now been extended by three hours in Baghdad, Diyala, Salaheddin and Babil provinces.
In the latest violence, a Shi'ite muezzin, who calls for prayers from mosques, was shot dead in the northern town of Tuz Khormatu overnight, and a car bomb attack targeted a mosque in the southern city of Basra.
Civil war fears
Iraqi President Jalal Talabani earlier warned that widespread rebellion could engulf his already war-torn country.
The attack on the 1,000-year-old Imam Ali al-Hadi mausoleum, one of the countries' main Shi'ite shrines, in Samarra, has been blamed on suspected al-Qaeda-linked militants.
The main Sunni political party has declared a boycott of talks with the Shi'ites on a government of national unity to protest reprisal attacks on its community threatening to derail negotiations.
An emergency meeting of national leaders called by Mr Talabani in a bid to restore calm was also boycotted by the Sunni Arab National Concord Front.
"Putting out the fire of sedition is a sacred duty and requires national unity," Mr Talabani told reporters after the meeting.
"The fire of sedition, when it breaks out, can burn everything in its path and spare no one," he warned.
Bodies dumped
Eighty bullet-riddled corpses were brought to the Baghdad morgue between Wednesday afternoon and Friday, the deputy director of the morgue, Kais Mohammed, told AFP.
Many of them, dumped in and around Baghdad, could not immediately be identified, but they were widely believed to be those of Sunnis.
Another 47 men, shot to death, were discovered alongside 10 burned out cars on the outskirts of Nahrawan, south-east of Baghdad, police said.
It is believed they were workers from a nearby brick factory.
One Sunni was also killed and two wounded in a drive-by shooting outside a Sunni mosque in Baquba, north-east of the capital, and a Sunni sheikh was shot dead in Hillah, south of Baghdad.
In other violence, at least 12 people were killed in a powerful roadside bomb attack in Baquba, 60km north-east of Baghdad.
Eight were Iraqi army soldiers and four civilians, police said, adding 20 people were wounded.
Journalists killed
Police also reported finding the bodies of three Iraqi journalists working for Dubai-based Arabiya satellite television who were kidnapped near Samarra last evening while reporting on the shrine bombing.
Following the bloodless bombing of the shrine, dozens of Sunni mosques have been attacked despite calls by Grand Ayatollah Ali al-Sistani, the leading Shi'ite cleric, to refrain from reprisals.
The attack on the Samarra shrine, where the Shi'ites buried two of their most revered imams in the 9th century AD, destroyed its dome.
Prime Minister Ibrahim al-Jaafari proclaimed three days of national mourning and Thursday was declared a public holiday in a bid to get people off the streets.
The US military meanwhile announced that seven of its soldiers died in rebel attacks, taking the total US military deaths in Iraq since the March 2003 invasion to at least 2,290, according to Pentagon figures.