Millions of Muslim worshippers across the world have taken part in processions and services this week to mark the holiest days in the Shia calendar, Tasua and Ashura.

Tasua and Ashura, the ninth and 10th days of the month of Moharram, are days of mourning in Shia Islam to commemorate the martyrdom of Imam Hussein, a grandson of the Prophet Mohammed.

Ashura involves reenactments of the Battle of Karbala in which Hussein died in 680 AD, and public processions of intense collective grief accompanied by poetry recitals and music during which worshippers wear black.

In pictures: Muslims around world mark Ashura with self-flagellation Show all 18 1 /18 In pictures: Muslims around world mark Ashura with self-flagellation In pictures: Muslims around world mark Ashura with self-flagellation Iraqi Shiite boys take part in a self-flagellation procession during the reenactment of the Battle of Karbala, as part of a parade in preparation for the peak of the mourning period of Ashura Getty In pictures: Muslims around world mark Ashura with self-flagellation The religious festival of Ashura, which includes a ten-day mourning period, commemorates the seventh century slaying of Prophet Mohammed's grandson Imam Hussein in Karbala in Iraq Getty In pictures: Muslims around world mark Ashura with self-flagellation A Pakistani Shiite Muslim boy is silhouetted against the sky as he flagellates himself with chains and blades in commemoration of Ashura, in Peshawar, Pakistan EPA In pictures: Muslims around world mark Ashura with self-flagellation Police officers watch as boys participate in self flagellation during the Shi'ite Muharram procession in Peshawar, Pakistan Reuters In pictures: Muslims around world mark Ashura with self-flagellation A group of men participate in self flagellation during the Shi'ite Muharram procession in Islamabad, Pakistan Reuters In pictures: Muslims around world mark Ashura with self-flagellation Afghan Shiite Muslims use chains and blades during ritual self-flagellation as part of Ashura commemorations at a mosque in Kabul Getty In pictures: Muslims around world mark Ashura with self-flagellation An Afghan Shiite Muslim bleeds after using chains and blades during ritual self-flagellation as part of Ashura commemorations at a mosque in Kabul Getty In pictures: Muslims around world mark Ashura with self-flagellation Ritual self-flagellation as part of Ashura commemorations at a mosque in Kabul Getty In pictures: Muslims around world mark Ashura with self-flagellation Ashura commemorations at a mosque in Kabul. Ashura mourns the death of Imam Hussein, a grandson of the Prophet Mohammed, who was killed by armies of the Yazid near Karbala in 680 AD Getty In pictures: Muslims around world mark Ashura with self-flagellation Iraqi Shiite Muslim men flagellate themselves in commemorations on the tenth day of the mourning period of Muharram, which marks the day of Ashura Getty In pictures: Muslims around world mark Ashura with self-flagellation A man's back is stitched up by medics after self flagellation during the Shi'ite Muharram procession in Islamabad, Pakistan Reuters In pictures: Muslims around world mark Ashura with self-flagellation Afghan Shiite Muslims watch as others use chains and blades during ritual self-flagellation as part of Ashura commemorations at a mosque in Kabul Getty In pictures: Muslims around world mark Ashura with self-flagellation Iraqi Shiite Muslim men are seen after flagellating themselves on the tenth day of the mourning period of Muharram, which marks the day of Ashura, in the holy city of Najaf Getty In pictures: Muslims around world mark Ashura with self-flagellation Iraqi Shiites take part in commemorations on the tenth day of the mourning period of Muharram, which marks the day of Ashura, in the holy city of Najaf Getty In pictures: Muslims around world mark Ashura with self-flagellation Knives are displayed for sale during commemorations for Ashura in Kerbala, Iraq Reuters In pictures: Muslims around world mark Ashura with self-flagellation Iraqi Shiite boys take part in a self-flagellation procession during the reenactment of the Battle of Karbala, as part of a parade in preparation for the peak of the mourning period of Ashura Getty In pictures: Muslims around world mark Ashura with self-flagellation A Pakistani Shiite Muslim flagellates himself with chains and blades a day ahead of the commemoration of Ashura, in Islamabad, Pakistan EPA In pictures: Muslims around world mark Ashura with self-flagellation Pakistani Shiite Muslims flagellate themselves during the Ashura procession in Peshawar. The Islamic month of Muharram marks the seven-century martyrdom of Prophet Mohammad's grandson Imam Hussein who was killed in battle in Karbala in Iraq in 680 AD Getty

In certain parts of the world, observers beat their chests and heads and flagellate themselves with chains or swords to echo his suffering.

The date is decided according to the new moon and is celebrated as a public holiday in Afghanistan, Iran, Iraq, Lebanon, Azerbaijan, Turkey, Bahrain and Pakistan. This year, it fell on 29-30 September - the culmination of four weeks of mourning.

The self-flagellation ritual, known as ‘Tatbir’ in Arabic and ‘Talwar Zani’ and ‘Qama Zani’ in south Asia, is a tradition that originates from southern Lebanon and Karbala, in southern Iraq, where Hussein died.

It has been banned or strongly discouraged by many governments and Shia religious authorities in recent years because of the associated health risks.

Iran's Supreme Leader Ali Khamenei issued a decree against the practice in 1994.

Hezbollah, the armed Lebanese Shia political organisation, has repeatedly asked devout Muslims who wish to mourn Hussein to donate blood to hospitals and blood banks instead.

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Nonetheless, in Nabatiyeh, in the south of the country, mourning processions organised by political group Amal drew thousands of worshippers who took part in the tradition left the streets covered in blood and the air smelling like iron on Sunday.

In Beirut’s southern suburbs tens of thousands of men and women took part in a bloodless procession which ended in a televised speech by Hezbollah chief Sayyed Hasan Nasrallah, in which he said that the recent Kurdish independence referendum in Iraq was the product of an American-Israeli conspiracy to further “divide the Middle East”.

The debate over whether Imam Hussein was entitled to the position of caliph through his bloodline is where the two branches of Sunni and Shia Islam originate, and as such the holiday is sometimes used as an excuse for violence because of the marked sectarian differences in how it is celebrated.