Article published on the 2009-07-02 Latest update 2009-07-02 13:46 TU
The blast took place at a congested junction in the city, which is home to the headquarters of the military and intelligence agencies and right next door to the capital, Islamabad.
Police say that two dozen members of a government department were on the bus at the time and believe that the explosion was a suicide attack.
Early reports put the death toll at six but police say that, after checking hospitals, they can only confirm one death and over 20 injured. Several other vehicles were seriously damaged.
About 2,000 people have been killed in bombings in Pakistan since July 2007 and US President Barack Obama sees the country as key to efforts to fight Al-Qaeda and the Taliban, who are also active in neighbouring Afghanistan.