Iraqi blast damages Shia shrine
Grand Ayatollah Ali al-Sistani, the spiritual head of Iraq's Shia Muslims, has called for a week of mourning.
Shias in Baghdad attacked at least five Sunni mosques in reprisal raids, with disturbances reported in other cities.
The BBC's Jon Brain in Baghdad says the attack was almost certainly designed to raise the existing tensions between the majority Shia and minority Sunni populations.
Shias distraught
The shrine is one of two tombs in Samarra for revered Shia imams, which attract pilgrims from around the world.
It was attacked one day after at least 22 people died when a car bomb exploded in a market in a Shia neighbourhood of southern Baghdad.