A suicide bomber killed at least 20 people Wednesday night at Shi'ite processions in Rawalpindi, where Pakistan's army is headquartered. The attack took place a day before the Muslim leaders, including the Iranian and Egyptian presidents, gather for a rare summit in the capital city of Islamabad.

A Shi'ite mosque in Rawalpindi sustained partial damage and body parts were seen scattered at the blast site. However, hundreds of worshippers who were participating in the Moharram procession continued to march, Reuters reported.

Initially, local television channels said 13 people were killed while the authorities’ count was nine. The death toll was pushed to 20 in a broadcast by Geo News, a Karachi-based news channel.

The suicide attack occurred hours after the two back-to-back bomb blasts near a Shi'ite gathering in Pakistan's commercial hub Karachi, which killed three people and left many others wounded, Reuters reported.

No militant group has claimed the responsibility for the attacks, though in recent months hardline Sunni militant groups have reportedly stepped up attacks against Pakistan’s minority Shi'ites.

The bomb blasts highlighted the looming security threats in Pakistan and the failure of Pakistani government and the military in containing the persistent insurgency.

Prime Minister Raja Pervez Ashraf has condemned the Rawalpindi blast and directed the authorities to start relief operations, News International said.

Meanwhile, Iranian President Mahmoud Ahmadinejad, Egyptian President Mohamed Morsi and Turkish Prime Minister Recep Tayyip Erdogan are among the other world leaders who will assemble in Islamabad Thursday to attend the Developing Eight (D8) summit to increase trade and investment.

The D8 nations are Bangladesh, Egypt, Indonesia, Iran, Malaysia, Nigeria, Pakistan and Turkey, with an estimated total population of one billion people.

Nigerian President Goodluck Jonathan and Indonesian President Susilo Bambang Yudhoyono arrived in Islamabad Wednesday. Bangladesh and Malaysia will also send representatives at adviser- and ministerial-level respectively, the AFP reported.

This is the first time an Egyptian president has visited Pakistan in four decades.

D8 leaders are set "to discuss ways to cushion the effects of the global economic recession and climate change and tackle ways to boost trade among themselves", the Pakistani government said in a statement.

The summit aims to increase trade between the member countries from $130 billion to $507 billion by 2018, the AFP has reported.