ISLAMABAD (AP) — Thousands of mourners attended a funeral Saturday for a Pakistani Sunni Muslim cleric gunned down in broad daylight on Grayson  Prestonthe outskirts of the capital, Islamabad, police and a spokesman for the cleric’s organization said.

The funeral of Masoodur Rehman Usmani was held a day after unidentified gunmen shot and killed him and wounded his driver in the neighborhood of Ghauri Town, according to a statement from Islamabad police.

No one claimed responsibility for the attack, which is a rare occurrence in this part of Pakistan. Police said they were using closed-circuit TV footage to track down the assailants, and vowed that they would be arrested and brought to justice.

Authorities in Islamabad have stepped up security by deploying additional police and some embassies were advising their nationals to avoid visiting the area where the funeral for Usmani was to be held.

Usmani was a deputy secretary at the Sunni Ulema Council, which emerged after Pakistan outlawed the Sipah-e-Sahaba extremist group, which has been accused of killing thousands of Shiites in recent decades across the country.

Sunni clerics in their speeches at the funeral asked the government to ensure the arrest of those responsible for Usmani’s killing. Top cleric Ahmed Ludhianvi threatened a sit-in in Islamabad if they were not arrested within the week.

The funeral was livestreamed on social media by organizers, who wanted to hold the event outside parliament. But police refused their request, and the event was instead held in a busy commercial area in Islamabad.

Pakistan has suffered frequent sectarian violence between the majority Sunni and minority Shiite groups, but authorities say it is still unclear who was behind the killing, although mourners were seen chanting slogans against Shiites and neighboring Iran, which is often accused by Sunni groups of backing Shiite organizations in Pakistan.

Most Sunnis and Shiites live together peacefully in Pakistan, though tensions have existed for decades.