State television reports said that a similar indefinite curfew would be imposed in the capital Baghdad.
Muqtada al-Sadr, a leading Shia cleric, called for three days of national mourning after the attack.
A senior spokesman for the political bloc loyal to al-Sadr called for calm but questioned how the bombing could have occurred.
"This is a terrorist act. How did the terrorists manage to go inside the shrine, plant the explosives and then blow it up, unless they have some people in the [US and Iraqi government] forces there helping them?" Abdul Mahdi al-Mutiri said.
He also said the group was suspending its participation in parliament in protest.
Grand Ayatollah Ali al-Sistani, Iraq's most senior Shia cleric, also called for restraint.
|Nuri al-Maliki blamed the bombing on al-Qaeda |
fighters and Saddam loyalists [AFP]
"He condemns the attack and urges calm and not to do acts of reprisal against Sunnis," Hamed Khafaf, Sistani's spokesman, said.
The US military said the fresh attack on the shrine might provoke another spike in sectarian violence.
"Based on the results of last year's attack, we are obviously watching it very carefully," Lt Colonel Christopher Garver, a US military spokesperson, said.
He said the military had not yet made an independent assessment of the latest incident.
In apparent retaliatory acts, four Sunni mosques in Iraq were attacked on Wednesday afternoon.
Three mosques were bombed in the town of Iskandiriyah, 60km south of Baghdad, police Lieutenant Mohammed Ruhemi said.
Meanwhile, a Sunni mosque in Baghdad's Bayaa neighbourhood was set on fire, local residents said.
Police said the Shia-dominated interior ministry had been responsible for security at the Askiriya mosque, after taking over from local security forces in April.
"Iraqi security forces have been protecting the shrine from all sides since April 1, and no one is allowed to approach it," Mahmoud Saleh, an Iraqi journalist, told Al Jazeera.
"The incident was shocking as people in Samarra did not expect such a thing to happen."
The bombing in Feruary 2006, which destroyed the mosque's golden dome, triggered nationwide Shia and Sunni sectarian clashes.
The Askariya contains the tombs of the tenth and eleventh imams, Ali al-Hadi, who died in 868, and his son Hassan Askariya, who died in 874.
Descendants of the Prophet Muhammad, Shias consider the two imams among his natural successors.