Millions of pilgrims have descended on the holy city of Karbala in central Iraq to mark Arbaeen, which commemorates the seventh century martyrdom of a central figure in Shiite Islam.

Iraqi state-run media said Wednesday that more than 22 million pilgrims have visited Karbala, setting a record for the annual pilgrimage. That number could not be independently verified.

Security has been tightened across Baghdad and mainly Shiite southern Iraq. Many pilgrims travel by foot to Karbala, and Shiite observances are often attacked by Sunni extremists.

The Islamic State group has repeatedly targeted Iraq's Shiite majority. The Sunni extremist group considers Shiites apostates who deserve death.

Iraqi authorities say last year's Arbaeen drew 17 million pilgrims.