Thousands of Iraqis have gathered at the al-Askari shrine in Samarra, north of Baghdad, where two men blew up the famous golden dome in a dawn raid.
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.
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.If things do break down, it will mean a more active role for Shiite-controlled Iran, which has already been reported as sending arms to Iraqi insurgents.