By Ahmed Saeed
KERBALA, Iraq (Reuters) - Millions of Shi'ite Muslims flocked to Iraq's holy city of Kerbala on Wednesday to commemorate Ashura, a religious day marking the slaying of Prophet Mohammed's grandson Hussein in the 7th century AD in a revolt against the Umayyad ruler Yazeed.
Visitors came from Pakistan, Afghanistan, Iran and the Gulf as well as Iraq, where Shi'ites form the majority community.
Mourners dressed in black and waving flags with the slogan "O Hussein" moved around his mausoleum, hitting their heads and chests in a show of sorrow at the suffering of the imam and his family.
Imam Hussein was killed on the site along with his half-brother Abbas, his son Ali Akbar and dozens of his warriors on the Day of Ashura. Other members of his family, including his sister Zainab and his daughters were taken as captives to Damascus.
The day marks Hussein's epic battle against Yazeed's troops, who outnumbered his forces and who deprived him and his family access to the waters of the Euphrates River to force his surrender.
This year's commemoration is held as Iraqi forces prepare an offensive on Mosul, the last city still under control of Islamic State, the hardline Sunni group that considers the Shi'ites as apostates.
Qais Al-Khazali, the leader of the Iran-backed paramilitary group Asaib Ahl al-Haq, compared those who killed Husein in Kerbala to Islamic State, which declared a caliphate in parts of Iraq and Syria in 2014.
"The liberation of Mosul will be the revenge against the killers of Hussein, because these (IS) are their grandsons," he said in a video posted on the Kurdish news website Rudaw.
(Writing by Maher Chmaytelli; Editing by Angus MacSwan)