Shia Muslims hold mourning ceremonies during the first 10 days of Muharram to remember Imam Hussein and his 72 companions, who were all martyred after fighting thousands of soldiers of the ruling tyrant of the time, caliph Yazid I in Iraq's Karbala desert in 680 AD, Presstv Reported.
Dunning black attire to show their sorrow, Iranians from all walks of life on Monday joined processions that started from the early morning and would last until about midnight.
During the ceremonies, eulogists recite the events of the Battle of Karbala and Islamic scholars elaborate on the underlying messages of Imam Hussein's stance against tyranny.
The highlight of the processions in Iran, however, is Ta'zieh, a national and religious dramatic musical performance that is mostly used to narrate the story of the martyrdom of Imam Hussein and his companions in Karbala.
Ta'zieh was registered on UNESCO’s list of the Intangible Cultural Heritage of Humanity in 2011.
Tasu'a is devoted to Abbas ibn Ali, Imam Hussein’s half-brother, who made great sacrifices to protect the Imam and his family and was eventually martyred on the 10th day of Muhrram, or Ashura, shortly before Imam Hussein himself.
He was martyred as he tried to bring water for women and children in Imam's camp, who had no water to drink for days due to a siege by the enemy.
Abbas' loyalty to Imam Hussein and his bravery in the face of Yazid's massive army is why Tasu'a is called the Day of Loyalty and Resistance in his honor.
He is buried in the city of Karbala in his own shrine, which is within walking distance from Imam Hussein's mausoleum.
Many Iranians also travel to Iraq to partake in mourning rituals at the holy shrines in Karbala, as well as in Najaf, where Imam Hussein's father, Imam Ali, is buried.
Mourning ceremonies are held in other Shia communities across the world, including in Lebanon, Bahrain, Azerbaijan, Pakistan, Afghanistan, India, Kashmir and Nigeria.
This year's processions in Kashmir are being held under a heavy-handed crackdown by Indian forces. New Delhi, after stripping the Muslim-majority Himalayan region from its autonomy, has announced a curfew and put Kashmir on a media blackout.
It is a similar story in Nigeria, where supporters of Sheikh Ibrahim al-Zakzaky, leader of the Islamic Movement in Nigeria (IMN), have pledged to brave the military crackdown and hold processions for Imam Hussein.
They have also pledged to stage massive protests demanding the release of Zakzaky, who has spent the past few years in Nigeria's prisons.
Islamic centers in the United States and some European countries also mark the occasion by holding special ceremonies.