Politicians around the world recoiled at the spectacle of mobs machine-gunning and torching mosques in Baghdad in an unprecedented eruption of violence by the Shiites against the country's former Sunni elite.
The violence was triggered by two explosions which brought down the gilded dome of the 1,000-year-old Imam Ali al-Hadi mausoleum in the northern town of Samarra, fanning fears of a sectarian war between Iraq's ruling Shiite majority and the ousted Sunnis.
US President George W Bush denounced the blast and pleaded for calm, as crowds attacked 27 Sunni mosques in Baghdad, killing six people, in enraged reprisal attacks over the destruction of the Shiite shrine.
"I ask all Iraqis to exercise restraint in the wake of this tragedy, and to pursue justice in accordance with the laws and constitution of Iraq," President Bush said in a statement.
"Violence will only contribute to what the terrorists sought to achieve."
Violent hotspots
In Baghdad, mobs killed three clerics and three worshippers in assaults on 27 Sunni mosques, an Iraqi security officer told AFP.
Crowds machine-gunned numerous religious sanctuaries and torched at least one, the officer added.
In the Shiite south, police reported that a crowd stormed the Basra offices of the Sunni-based political Islamic Party, killing two people and wounding 14 others while a mob wounded a guard in an attack on the Islamic Party offices in Nassariyah.
"We ask the Marjaiya (Shiite grand ayatollahs) to intervene before it is too late," the Islamic Party's chief Tareq al-Hashimi told reporters.
Mobs nationwide ignored a public appeal by Iraq's top Shiite religious authority, Grand Ayatollah Ali al-Sistani, who urged his community to remain calm and to refrain from seeking vengeance.
The office of Iraqi Prime Minister Ibrahim Jaafari, who proclaimed three days of national mourning, later announced that three suspects had been arrested in connection with the dawn Samarra bombing.
News of the arrests was flashed on local television broadcasts in a bid to ease tensions.
Prime Minister Jaafari called on Iraqis to denounce sectarian attacks and "close the road to those who want to undermine national unity".
Waving the green flags of Islam and the national Iraqi colours, thousands of Shiites had earlier taken to the streets of Samarra, 125 kilometres north of Baghdad, vowing to punish those responsible for the attack.
Shops closed and muezzins recited prayers from the loudspeakers of nearby mosques and blamed the United States for the turmoil, saying "God is Great, death to America which brought us terrorism."
A few hours later, tens of thousands rallied in Shiite districts of Baghdad and in the Shiite holy cities of Karbala and Najaf, in southern Iraq.
International reaction
British Prime Minister Tony Blair expressed alarm that the attack could be a catalyst for broader conflict and sabotage Iraqi efforts to form a national unity government two months after national elections.
"The perpetrators of this attack had one motive and one alone -- they want to cause strife and violence between Sunni and Shia, to derail democracy currently taking hold in Iraq," Mr Blair said in a statement.
UN Secretary General Kofi Annan said in a statement the attack was "clearly aimed at provoking sectarian strife and undermining further the peace and stability of Iraq".
In Jordan, King Abdullah II warned the destruction of the mosque "...was aimed at sowing and fanning sectarian strife among the Iraqi people".
"What happened is an attempt to disrupt the efforts being made to enhance national unity ... rebuild the nation and achieve a prosperous future for Iraq," the king said in a message to Iraqi President Jalal Talabani.
However Shiite-led Iran, expressing outrage over the bombing, pointed the finger at Washington over the bombing.
Supreme leader Ayatollah Ali Khamenei, declaring a week of national mourning, called on Iraq's newly empowered Shiite majority not to resort to revenge attacks against the ousted Sunni Arab elite.
"This is a political crime, which must be tracked back to the intelligence services of the Zionists and the occupiers of Iraq," the Iranian media quoted Ayatollah Khamenei as saying.