Curfews have been imposed in Baghdad and in the northern Iraqi town of Samarra following this morning's Shia mosque attack.
Two minarets of the al-Askari mosque in Samarra were destroyed in separate bomb blasts this morning.
Authorities are attempting to avoid violence of the sort that followed last year's attack on the same building.
The building, also referred to as the Golden Mosque, is one of four major Shia shrines in Iraq and is situated in a predominantly Sunni town.
The two gold-covered minarets collapsed after being blown up within minutes of each other.
On 22 February last year, alleged al-Qaeda militants bombed the shrine in an attack which triggered nationwide Shia and Sunni sectarian clashes that are still continuing.
The dome of the mosque was destroyed in that attack, and no reconstruction work has taken place.
The area had been heavily fortified since last year's attack, but it is understood there had been a row between elements of the security forces responsible for guarding the site.
The Sunni stronghold of Samarra has long been a centre of the armed insurgency against US-led forces in Iraq.
The al-Askari shrine is revered by Shias. Two of the 12 revered Shia imams are buried in the shrine: Imam Ali al-Hadi, who died in 868, and his son, the 11th imam, Hasan al-Askari who died in 874.
Some Shias believe the 12th imam, Imam Mehdi, known as the hidden imam, went into hiding in the complex in 878 and was not seen again.
