According to Aljazeera's correspondent, at least 70 people were killed on Tuesday in five explosions that rocked the holy Shia city of Karbala.
Witnesses reported seeing limbs and blood strewn across a street in central Karbala after the blasts. People were panicking and crying, witnesses said.
More than two million people were estimated to have been in Karbala, packing the streets for the Ashura ceremony, one of the holiest in the Shia calendar.
The explosions were caused by rockets fired at the shrine of Abbas from the north of the town, police said.
"The blasts were caused by at least six rockets, fired from the Hyabi zone, in the north of the town, on the mausoleum of Abbas," police spokesman Rahim Mishawi told AFP.
The attacks were carried out on Ashura, the holiest day of the Shia calendar, when believers mourn the slaying of al-Husain, grandson of the Prophet Muhammed, more than 1300 years ago. Abbas was the half-brother of al-Husain.
"Ambulances, and even civilian cars, were transferring casualties to hospitals," Aljazeera's correspondent reported.
"A huge number of Iraqi policemen and Polish troops were already deployed in the city of Karbala," said the correspondent.
"Security measures have been highly stepped up during the last couple of days in Karbala and Baghdad to prevent such attacks," he added.
Shia the world over mark
al-Husayn's death every year
Also on Tuesday, at least 75 people were killed when four blasts rocked the holiest Shia mosque in the north of the Baghdad, Aljazeera's correspondent reported.
Witnesses said the explosion was caused by a rocket fired at the crowd at al-Khadhimiya mosque.
Interviewed by Aljazeera, Shaikh Jawad al-Khalisi, the Imam of al-Kadhmi mosque, lashed out at the "US forces' intervention."
"The US intervention in the country's security is against the will of the Iraqi people," al-Khalisi told Aljazeera.
"The people behind the explosions seek to create sectarian strife between the people of Iraq. However, Iraqis will never allow this to happen."
Aljazeera's correspondents and camera crew in Baghdad were slightly injured when the car they were travelling in was hit by a bomb, perhaps in reaction to the blasts.
"We doubt that the attackers were Muslims. Any body who kill innocent souls during this occasion is not related to Islam."
Statement from the Association of Muslim Scholars (AMS)
The Sunni Association of Muslim Scholars (AMS) condemned the blasts.
In a statement sent to Aljazeera, AMS blamed the US for failing to provide safety measures and "for destroying the national security departments."
AMS also said the attacks were "skillfuly designed to incite sectarian conflict during this difficult time that Iraq is going through".
AMS was also sceptical of the possibility that the attacks were carried out by Muslims.
"We doubt that the attackers were Muslims. Anybody who kills innocent souls during this occasion is not related to Islam."
The blasts, coming at such a sensitive and holy time for Iraq's majority Shia, threaten to increase sectarian tensions in the country.
The day of Ashura has religious significance for Sunnis for a different reason, but Shia mark the day by beating their heads and chests, and gashing their heads with swords, to show their grief and echo the suffering of al-Husayn, who was beheaded by his enemies.
Hundreds of thousands of Shia had been marching through
the streets of Karbala, chanting and flagellating themselves.
"I saw Iranians beating themselves, and then I heard a big
explosion. The blast seemed to come from a cart they were
pushing," said Wisam Hadi, a witness in Karbala.
Thousands of Shia have flooded across the Iranian border for the events.
Thousands more were on the streets of Kadhimiya, the Shia district of Baghdad targeted by the blasts.