The ear-splitting drum reverberates as rhythmic poems and chants are recited to venerate the Prophet Mohammed’s grandson Imam Hussein.
Teenagers, in matching black shirts and white caps, stand in lines, shouting back rhymes and flogging themselves with chains in unison.
Others gather around them, swaying to the rhythm and slamming their chests with their fists.
It is one of the key centuries-old rituals during the annual solemn mourning period for millions of Shiite Muslims in Iraq and beyond that is usually performed in streets and around revered shrines.
But this year it is taking place in the cavernous, marbled entrance hall of Baghdad's parliament building, the setting that Shiite cleric Moqtada Al Sadr has chosen for the latest bout of the country’s prolonged political stalemate over forming a new government.
In 680, Imam Hussein had revolted against the Damascus-based second Umayyad caliph, Yazid Ibn Muawiyah, moving from Madinah to outside Karbala in modern day Iraq, where a battle took place.
To quell the revolt, Yazid sent an army that slaughtered Imam Hussein and most of his family in an area called Al Taf.
His death was a defining moment in Islamic history and its commemoration has become the most impassioned event for Shiite Muslims around the world.
In Iraq and beyond, millions of Shia Muslims observe with different rituals the death anniversary that falls on the tenth day of the Islamic month of Muharram, known as Ashura in Arabic, as well as the 40th day of his death in the following month of Safar.
Muharram started on Sunday in Iraq.
The narrative of the Al Taf Battle is so powerful, it is observed by Shiites with both faith and fury. For them, Imam Hussein is a symbol of reform and a strong voice against oppression.
Revolving around these themes, Mr Al Sadr, himself a descendant of Prophet Mohammed, kindled his recent protests to keep emotions high among his supporters while facing his rivals.
“Oh Hussein, our revolution is an extension of yours,” chanted a cleric as a procession went on inside the parliament building early this week. “We swear by your wounds, we will not bow.”
A large Iraqi flag hung above the faithful, with the famous saying for Imam Hussein written on it: “I never revolted in vain, as a rebel or as a tyrant, but I rose seeking reform for the nation of my grandfather Mohammed”.
Mr Al Sadr emerged as a powerful religious leader after the 2003 US-led invasion that toppled Saddam Hussein’s Sunni-led regime and brought Iraq's majority Shiites to power.
Shortly after the invasion, he formed a militia that attracted thousands from his wide support base, which consists largely of impoverished Iraqis. They fought fierce battles with US and British troops, particularly in Iraq's southern cities including Najaf, Karbala, Basra and Al Amarah.
When the country plunged into sectarian warfare between 2006 and 2008, his Mahdi Army militia was accused of killing minority Sunnis. Politicians linked to the Sadrist Movement are also blamed for contributing to Iraq's environment of endemic corruption.
Mr Al Sadr has tried to distance himself from violence during the period of sectarian chaos, claiming that members of the Mahdi Army who committed abuses were not official members of the group.
But in recent years, he has reinvented himself, presenting a picture of a populist leader who seeks reform through fighting corruption and sectarianism, while forming alliances with prominent Sunni politicians.
These are the themes his political group used in their campaign for October's elections after which they emerged as the clear winner with 73 seats in Parliament out of the 329.
But in June, Mr Al Sadr acknowledged his failure to form a majority government with other leading parties, including Sunnis and Kurds and ordered his MPs to resign. In returned, the Iran-backed Co-ordination Framework — whom Mr Al Sadr wanted to sideline, took the lead in forming the government.
As Mr Al Sadr’s followers were knocking down blast walls around the Green Zone on Wednesday, breaking into the parliament building to stop his rivals meeting to form the next government, he wanted them to remember Imam Hussein's revolution.
“The revolution of Al Taf has been the lighthouse for the free people around the world,” he said in a tweet. “A lot of revolutionaries are inspired by it to reject injustice, falsehood and corruption..”
When the protesters returned to Parliament on Saturday, he ordered them to stage an open-ended sit-in until his demands for overhauling the political system and changing the constitution were met.
His aids called on volunteers — who every year set up tents to offer free food and drinks to mourners during Muharram, in what is known as Mawakib, to serve the protesters.
Hours later, dozens of tents were erected and started to cook in large pots, used during Muharram. Posters for Imam Hussein hung next to Mr Al Sadr’s at these tents with text praising both men. Mourning processions are held daily when night falls.
For the tribal leader Kadhim Al Timimi, Mr Al Sadr “made a good choice to coincide the protest with the revolution of his grandfather Imam Hussein when he fought corruption and injustice”.
“Today, we ask God for the sake of Imam Hussein and his family to support the revolution of Iraq against injustice and corruption, to eradicate the corrupt and put them on trial,” Mr Al Timimi said.