A truck bomb killed 70 as a petrol station where buses packed with Shiite pilgrims returning from the Arbaeen commemoration in the Iraqi city of Karbala were parked, security officials said.
Several Iranian nationals -- the largest contingent of foreigners at the Arbaeen pilgrimage that ended on Monday -- were among the victims, the Joint Operations Command (JOC) said.
"At least seven buses with pilgrims were inside the petrol station at the time," a police lieutenant colonel told AFP.
The blast struck in the village of Shomali, 120 kilometres (75 miles) southeast of the capital Baghdad and around 80 kilometres (50 miles) from Karbala.
There was no immediate casualty toll but the JOC said "several civilians were killed and wounded, including Iranians".
"Those buses were loaded with Iranians, Bahrainis and Iraqis. Ambulances and civil defence are on their way to the site," a police intelligence source told AFP.
A Shomali resident said the petrol station was on the main motorway between Baghdad and the southern port city of Basra.
"There were Iranians but also lots of people from Basra and Nasiriyah," Mousa Omran told AFP, referring to another southern city.
Between 17 and 20 million people visited Karbala, home to the mausoleum of Imam Hussein, for Arbaeen, which is one of the world's largest religious events.
The days-long final phase of the pilgrimage sees huge numbers of pilgrims walk long distances to reach Karbala.
According to the Iraqi authorities, around three million Iranians were among this year's visitors. Many of them stay a few days longer to visit the shrine city.
Iraq's security forces were on high alert for the duration of the pilgrimage, seen as a major potential target for the Islamic State group.
The jihadist organisation, which is currently trying to defend its last major Iraqi bastion of Mosul against a massive offensive, has claimed countless such bombings.
Observers had feared the group, whose cross-border caliphate is crumbling under the pressure of multiple military operations backed by Iran and the West, would seek to attack Karbala.
Around 25,000 members of the security forces were deployed in and around the city last week to protect the pilgrims and the shrine but some have since returned to the front lines.
Thursday's attack marred an Arbaeen commemoration that had passed with fewer attacks than in previous years.
Source: National News Agency