More than two million pilgrims have gathered in the Iraqi city of Karbala to mark one of the holiest religious ceremonies on the Shi'ite Muslim calendar - Arbaeen.

The celebration reaches its climax Thursday with pilgrims gathering to pay homage at the burial ground of the Prophet Muhammad's grandson Imam Hussein.

Many of the pilgrims made the journey by foot from the capital, Baghdad, amid tight security.

Iraqi police said one Shi'ite pilgrim was killed Wednesday when a roadside bomb exploded near a bus in the Iraqi capital.

Earlier this week, a series of of attacks on pilgrims killed at least 63 people and wounded more than 100 others.

Some information for this report was provided by AFP and AP.