Thousands of people have mourned the 31 people killed in a deadly suicide bombing inside a Shiite mosque. An offshoot of the so-called "Islamic State" group claimed responsibility for the attack.

Thousands of mourners gathered in Pakistan's capital on Saturday for a somber mass funeral honoring the victims of a suicide bombing at a Shiite mosque the previous day.

At least 31 people were killed in the blast and around 170 were wounded, as authorities vowed to intensify operations against Islamist militants.

The attack, claimed by an affiliate of the so-called "Islamic State" group, has heightened fears of escalating sectarian violence in the heavily guarded city.

What Happened at the Mass Funeral?

Tens of thousands of people attended the funeral prayers in an open area near the mosque, with many openly weeping and beating their chests in traditional Shiite lamentation.

Around 20 coffins were carried away for burial, while additional prayers were held in Dera Ismail Khan for two other victims.

Shops shuttered across Islamabad, streets were deserted and unions called for widespread protests.

"What happened yesterday has left us extremely angry and deeply hurt," said Bushra Rahmani, whose brother was among the wounded.

Ashiq Hussain, grieving the loss of his 21-year-old nephew, lamented, "Our family is broken. What sin did this young man commit?"

Security was significantly bolstered, with police checkpoints on major roads and elite commandos guarding key sites amid concerns of further attacks.

How Did the Attack Unfold?

The bombing occurred during Friday prayers at the Khadija Tul Kubra Imambargah mosque, on the outskirts of Islamabad.

According to local administration chief Irfan Memon, a suicide bomber opened fire on security guards at the main gate before detonating an explosive vest at the inner entrance. As well as 31 people killed, around 170 others were wounded.

Witnesses described a chaotic scene, with gunfire exchanged with volunteer security personnel, followed by a massive explosion.

Unverified videos circulating on social media showed bodies near the mosque's front gate and the injured, including children, being rushed to ambulances amid screams and weeping relatives.

IS Affiliate Claims Responsibility

The regional IS affiliate, known as Islamic State in Pakistan, claimed responsibility for the assault in statements posted on Telegram and its Amaq News Agency.

The group described the attack as targeting "infidel" Shiites, accusing them of providing recruits to rival Shiite militias fighting IS in Syria.

Shiites, comprising 10 to 15% of Pakistan's 241 million predominantly Sunni population, have long been targeted by groups like IS and Tehreek-e-Taliban Pakistan.

The attack marks the deadliest incident in Islamabad since the 2008 Marriott hotel bombing, which killed 63 people, and follows a November suicide blast outside a court that claimed 12 lives.

Authorities Detain Suspected Mastermind, Accomplices

Authorities moved swiftly in response with overnight raids in Peshawar, Nowshera, and Khyber-Pakhtunkhwa, leading to the arrest of several suspects, including the bomber's family, according to security sources.

One militant and at least two law enforcement officers were killed during the operations, with three more agents injured.

State-run media reported the detention of an alleged Afghan IS mastermind, claiming the plot originated in Afghanistan. Pakistan's Interior Minister Mohsin Naqvi also said four people accused of helping the suicide bomber had been arrested.

Pakistani Prime Minister Shehbaz Sharif vowed to bring perpetrators to justice, expressing gratitude for international condolences from the US, Russia and the EU.

Pakistan has witnessed a surge in Islamist violence since the Taliban's 2021 takeover in Afghanistan.

A separatist group launched a series of coordinated assaults last week in Balochistan, which claimed the lives of 36 civilians and 22 security personnel, followed by operations killing nearly 200 militants.