A roadside bomb struck an election convoy of Pakistan's main opposition party on Tuesday, killing four people, police and officials said, the third deadly attack on political targets in as many days. Sanaullah Zehri, the head of the Pakistan Muslim League-N in southwestern Baluchistan province, was on the campaign trail for the May 11 general election when the blast hit his convoy in Khuzdar district, 350 kilometres (220 miles) south of provincial capital Quetta. Zehri survived the attack but his son, brother, nephew and their guard were all killed, officials said. "An improvised explosive device went off as Zehri, leading a convoy of more than 20 vehicles, left his home to campaign in Khuzdar," provincial home secretary Akbar Durrani told AFP. "Zehri survived as his car had already passed when the device detonated." A local police official confirmed the attack, saying that at least four other vehicles in the convoy were damaged. No one has immediately claimed responsibility for the attack, but the Pakistani Taliban have said they were behind two deadly roadside bomb attacks in the northwest on Sunday. Zehri survived a murder attempt two years ago that was claimed by the rebel Baluch Liberation Front. The campaign for Pakistan's historic general election, which should mark the first time a democratically elected government hands over to another after completing its five-year term, has already been blighted by violence. On Monday gunmen killed two election campaigners for an independent candidate running in the lawless northwestern tribal regions, a day after a roadside bomb in the Swat valley killed a local leader of the Awami National Party (ANP). The ANP ruled the northwestern Khyber Pakhtunkhwa province from 2008 until assemblies were dissolved last month for elections. The Pakistani Taliban have assassinated a number of senior ANP figures in recent years and have threatened to attack secular parties running in next month's polls. On Thursday a grocer standing in the polls for the secular Muttahida Qaumi Movement (MQM) was shot dead in a drive-by killing in the southern city of Hyderabad, the first candidate to die in the election campaign. Pakistan's caretaker interim prime minister Mir Hazar Khan Khoso ordered an immediate tightening of security for all candidates in the wake of that shooting.