Taliban and al Qaeda-linked violence since the 9/11 attacks and comes during the holy month of Muharram, a magnet for sectarian attacks.
Islamabad rarely hosts major international gatherings because of troubles with violence and its reputation as a hub for Islamist extremism in the northwest.
Thousands of extra police and paramilitaries will deploy in Islamabad for the Developing Eight summit, which brings together Egypt, Bangladesh, Indonesia, Iran, Malaysia, Nigeria, Pakistan and Turkey.
Iranian President Mahmoud Ahmadinejad, Egyptian leader Mohamed Morsi and Turkish Prime Minister Recep Tayyip Erdogan, key players in the Middle East, are among those expected to attend the D8 summit.
In the southwestern city of Quetta, bombers hit an army vehicle escorting children home from school, killing four soldiers and a woman, police said.
In northwest Pakistan, five policemen were killed in two other attacks.
More than 20 people were wounded when the Quetta bomb, planted on a motorcycle, was detonated by remote control, said Quetta 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.
He said 21 people were wounded, including three soldiers and 18 civilians.
"Six or seven of them are in a serious condition," Shakeel added.
Akbar Hussain Durrani, the home secretary of Baluchistan province, said two children aged eight and 11 who were passing by were among the wounded.
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."
Quetta is more than 400 miles (640 kilometres) southwest of Islamabad, where Pakistan has stepped up security significantly for the summit.
There was no immediate claim of responsibility for Wednesday's attack but Quetta and its province Baluchistan are frequently hit by bomb attacks.
The oil- and gas-rich area borders Iran and Afghanistan, and suffers from sectarian violence, attacks by Taliban militants and a tribal insurgency.
Baluch rebels rose up in 2004, demanding political autonomy and a greater share of profits from the region's mineral resources.
In northwest Pakistan, four police died when gunmen ambushed a routine patrol in the Jani Khel area of Bannu district, Nisar Ahmed Tanoli, the local police official, told AFP.
A roadside bomb in Shangla district also killed one police official and injured four others, according to police.
Local police official Saeedur Rehman said the target was a police van on a patrol 70 kilometres east of Mingora, the main town in the Swat region, where the army said it had defeated a Taliban insurgency in 2009.
Two people were also left dead on Wednesday 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 later, there was another blast that wounded seven people including journalists, policemen and paramilitary soldiers who had gathered there after the first explosion blast, said Javed Odho, another senior police official.