A group of gunmen, including two suicide bombers, stormed a coffee shop in a town north of Baghdad, killing at least 14 people and wounding 15, understood to be Real Madrid supporters.
Within hours, the Islamic State group posted a statement online claiming responsibility for the attack, which took place shortly after midnight on Thursday in Balad, 80km north of the Iraqi capital, news.com.au reported.
President of the Madrid supporters club, Ziad Subhan, said: "A group of Islamic terrorists, from ISIS, came into the cafe, armed with AK-47s, shooting at random at everyone who was inside".
When asked about the motive for the attack, the president said: "They don't like football, they think it's anti-Muslim. They just carry out attacks like this. This is a terrible tragedy".
Real Madrid released a statement expressing its "great sadness" over the attacks, adding that the team's players will wear black armbands during a game this weekend in tribute.
"Football and sport shall always be spaces in which to come together and in which harmony and peace reign and with which no form of barbaric terrorism will be able to compete," the club said.
The attack came on the heels of a two-day wave of bombings in Baghdad that killed nearly 100 people - attacks that have also been claimed by the Islamic State group. The deadliest struck the sprawling Shiite neighbourhood of Sadr City in northeast Baghdad on Wednesday, killing 63 people.
According to Iraqi officials, three gunmen armed with machine guns opened fire into the crowded Balad cafe and once police arrived at the scene, two of the attackers detonated their suicide vests. The officials spoke on condition of anonymity because they were not authorised to talk to the media.
Later on Friday, Iraq's top Shiite cleric, Grand Ayatollah Ali al-Sistani, condemned the wave of attacks and said the government is ultimately responsible for such security breaches, accusing the country's politicians of "closing their ears to their advisers."
ISIS attacked the headquarters of Real Madrid supporter club in Iraq, killing 14 people and injuring 28 more. R.I.P. pic.twitter.com/EOrua5gHVZ— Cristiano Ronaldo (@Cr7Prince4ever) May 13, 2016
Al-Sistani's words were relayed by his aide Ahmed al-Safi in a televised speech from the holy city of Karbala following Friday prayers.
Over three months ago, al-Sistani suspended his weekly political sermons after his repeated demands that the country's politicians tackle corruption went unheeded.
The IS bombings this week exposed lingering gaps in Baghdad's defences, which are manned by an array of security agencies and militias that don't always co-operate. They also point to the resilience of the extremist IS group, which has increasingly resorted to bombings in civilian areas far from the front lines as it has lost some territory to Iraqi forces backed by US-led air strikes.
On Thursday evening, hundreds took to the streets in Baghdad's Sadr City to demand government accountability for the security breaches. Protesters carried signs calling for the interior minister to resign while others called for the minister of defence and Prime Minister Haider al-Abadi to also step down.
Anti-government protests first erupted last summer as temperatures soared and millions were left without electricity. While al-Abadi proposed a series of government reforms in August 2015 that he claimed would combat corruption, very little has been implemented. Repeated delays in Iraq's parliament sparked another wave of protests this year, led by influential Shiite cleric Muqtada al-Sadr.
In late April the cleric's supporters stormed Baghdad's heavily fortified Green Zone and the parliament building.
Iraqi officials and analysts warn that the deepening political crisis may be distracting Iraq's security forces from the fight against IS. The Iraqi government claims IS only occupies 14 per cent of the country's territory after a string of battlefield losses, but the extremist group still controls key border areas between Iraq and Syria as well as Iraq's second largest city of Mosul.