A suicide bomber attacked a Shiite Muslim procession in Pakistan Wednesday, killing 16 people in the most deadly incident on a day of violence that left at least 28 dead on the eve of a major international summit. The blast in Rawalpindi near the capital Islamabad came after a series of earlier attacks across the nation, a stark reminder of the security challenges facing a country plagued by Taliban and Al-Qaeda-linked violence. The Rawalpindi police chief said the blast in the city during the holy month of Muharram -- a magnet for sectarian attacks -- occurred when a suicide bomber entered the procession and security officials were checking him. \"The suicide bomber blew himself up when the security officials were checking (his body). We had prior information about the attacks and were fully alert,\" said Azhar Hameed Khokhar. \"The total number of dead people has now reached 16. Some 32, including nine children, have got injured,\" Waqas Rehman, a spokesman for the Rescue 1122 service, told AFP. Another police official, Muhammad Haroon, told AFP that the attack took place when the procession was almost 500 metres (yards) from the mosque where it was heading. In the southwestern city of Quetta, bombers hit an army vehicle escorting children home from school, killing four soldiers and a woman, police said. More than 20 people were wounded when the bomb, planted on a motorcycle, was detonated by remote control, said city police chief Hamid Shakeel. \"The target was an army vehicle which was escorting a school bus carrying children of local army officers from different schools,\" he told AFP. \"Six or seven of them (the wounded) are in a serious condition,\" Shakeel added. Witnesses said the motorbike appeared to have been parked near shops to avoid any suspicion in the Shahbaz Town neighbourhood near prestigious private schools. \"I was returning to my shop after saying prayers in a nearby mosque,\" said shopkeeper Mohammad Talib, 45. \"Soon after, I heard a huge blast. There was dust and smoke. I saw an army vehicle in flames. Shards of glass were littered on the road. There was panic, people were screaming, others were fleeing the area.\" Fruit vendor Abdul Karim, 30, said the army vehicle took the same route every day after school. \"After some time police and FC (Frontier Corps paramilitary) troops arrived. They fired in the air to scare people away. Soon shops were closed and people emptied the area.\" Two people were killed in the country\'s largest city, Karachi, as a bomb-laden motorcycle collided with a rickshaw near a Shiite Muslim mosque in the Orangi neighbourhood, city police chief Iqbal Hussain told AFP. Minutes after the Karachi attack, there was another blast that wounded seven people including journalists, policemen and paramilitary soldiers who had gathered after the first explosion, said Javed Odho, another senior police official. In northwest Pakistan, four police died when gunmen ambushed a routine patrol in Bannu district, Nisar Ahmed Tanoli, the local police official, told AFP. And a roadside bomb in Shangla district killed another police official and injured four others, according to police. Thousands of extra police and paramilitaries will deploy in the city for the Developing 8 (D8) summit, which starts on Thursday, bringing together Egypt, Bangladesh, Indonesia, Iran, Malaysia, Nigeria, Pakistan and Turkey. Islamabad rarely hosts major international gatherings because of its ongoing struggles with Islamic extremism. Iranian President Mahmoud Ahmadinejad and Turkish Prime Minister Recep Tayyip Erdogan are among those expected to attend.