By Kareem Raheem
BAGHDAD, June 13 (Reuters) - Bombers struck at Shi'ite pilgrims celebrating a religious festival in Baghdad and across Iraq on Wednesday, killing more than 70 people in one of the bloodiest days since the last U.S. troops withdrew from the country in December.
The bombings appeared to be the work of Sunni insurgents who often hit Shi'ite targets to try to reignite the intercommunal violence that killed tens of thousands of people in 2006-2007.
With the government's Sunni, Shi'ite and ethnic Kurdish parties already locked in a crisis that threatens to shatter their delicate power-sharing agreement, the attacks revived fears that Iraq risked sliding back into sectarian bloodshed.
It was the worst day of violence since early January, when four bombs in Baghdad killed 73, and the latest in a spate of bombings on Shi'ite religious sites.
At least 30 people were killed when four blasts hit pilgrims across Baghdad as they marched through the city to mark the anniversary of the death of Shi'ite imam Moussa al-Kadhim, a great-grandson of Prophet Mohammad.
One car bomb exploded outside a Baghdad Shi'ite mosque while another blast tore into groups of pilgrims as they rested at refreshment tents along the route to a shrine in Kadhimiya district.
"A group of pilgrims were walking and passed by a tent offering food and drinks when suddenly a car exploded near them," said Wathiq Muhana, a policeman whose patrol was stationed near the blast in central Karrada district.
"People were running away covered with blood and bodies were scattered on the ground," he said.
In a separate attack in the mainly Shi'ite southern city of Hilla, police said two simultaneous car bombs, including one detonated by a suicide bomber, exploded outside restaurants used by security forces, killing 22 people.
"When a minibus packed with policemen stopped near the restaurants, a car exploded near the bus," said Maitham Sahib, owner of a restaurant in Hilla near the blast. "It's heart breaking. It is just sirens, and screams of wounded people."
Iraq's renewed violence and political tensions will be closely watched by Sunni Gulf neighbours, and their rival, Shi'ite power Iran, who have meddled in Baghdad's politics in the past as they compete for regional influence.
In total, more than 21 bombs exploded on Wednesday in Baghdad and the southern Iraqi cities of Kerbala, Balad, Haswa, which are predominantly Shi'ite areas that have been targeted before by Sunni Islamist insurgents.
Just as the pilgrims began arriving in Baghdad on Sunday, at least six people were killed and 38 wounded when two mortar bombs struck a packed square in Baghdad's Kadhimiya district.
Earlier this month, 26 people were killed and more than 190 wounded when a suicide bomber detonated an explosive-rigged car outside a Shi'ite religious office in Baghdad, an attack claimed by al Qaeda's Iraqi affiliate, Islamic State of Iraq.
While violence has fallen sharply since the height of the war that followed the 2003 U.S.-led invasion that toppled Saddam Hussein, insurgents are still potent. Large bombings generally still hit once a month, usually on security forces, government offices or Shi'ite targets.
But since December when the last U.S. troops left, political tensions have also been on the rise.
Shi'ite Prime Minister Nuri al-Maliki is fending off attempts by Sunni, Kurdish and some Shi'ite rivals to organise a vote of no confidence against him. Critics accuse him of failing to fulfill promises to share government posts among the blocks.
Many Iraqi Sunnis fear Maliki is slowly sidelining them from the political process and trying to consolidate his own alliance's Shi'ite power at their expense.
Baghdad's central government is also caught in a long-running fight with the autonomous Kurdistan region over disputed land and oil claims.
Kurdistan, which already has its own government and armed forces, but relies on Iraq's oil revenues, has halted its own crude exports and hinted at a full breakaway from Baghdad in protest over what they say is Maliki's authoritarian style. (Additional reporting by Ahmed Rasheed in Baghdad, Habib al-Zubaidi and Ali al-Rubaie in Hilla, Jamal al-Badrani in Mosul and Mustafa Mahmoud in Kirkuk; Writing by Patrick Markey; Editing by Angus MacSwan)