Heavy security was deployed ahead of the ceremony in Karbala [Reuters]

Two bomb blasts near Iraqi holy city of Karbala have killed at least 18 people and injured scores of others taking part in the annual Arbain religious ceremony.

The explosions came as hundreds of Shia pilgrims gathered for religious rituals marking the end of a 40-day mourning period for Imam Hussein, a grandson of the Prophet Mohammad.

The first bomb exploded in a car park on the outskirts of the city where some of the pilgrims had parked their vehicles.

A few hours later a second car bomb exploded about 10km north of the city.

The explosions occurred despite the deployment of 120,000 police and soldiers across the country for the ceremony. "It's difficult for us to control every inch of the province. There are waves of pilgrims," an army captain said.

Iraq has been rocked by a series of blasts in recent days ahead of the culmination on Tuesday of Arbain.

More than 100 pilgrims, police recruits and police have been killed in an area ranging from Karbala to mainly Sunni areas north of Baghdad, the capital.

Although violence in Iraq has dropped sharply since its high point three years ago, the country is still plagued by small-scale attacks that have kept the nation on edge and raised doubts about the capabilities of its security forces.