Police said a remote-controlled explosive device was used in the attack.
Six persons were killed and 21 injured by the explosion, which caused extensive damage to the office of Syed Noor Akbar, an independent candidate contesting polls to a parliamentary seat in Orakzai tribal region.
Police said Akbar is a member of the minority Shia sect and this could have been a motive for the attack.
Hours later, another bomb went off outside an election office in Maqsoodabad, a suburb of Peshawar, the capital of Khyber-Pakhtunkhwa.
Three persons were killed and 10 others, including three children, injured, police said.
The blast targeted the office of Nasir Khan Afridi, an independent candidate contesting polls to a parliamentary seat in the lawless Khyber tribal region.
Police officials said the bomb was attached to a bicycle left outside the office. The office and two cars were damaged. Footage on television showed debris and election materials strewn on the road outside the office.
In the evening, two persons were killed and five injured when a bomb attack targeted an election meeting by Awami National Party candidate Ameer Rehman at Swabi in Khyber-Pakhtunkhwa.
Police said a remote controlled device was used in the attack.
Rehman escaped unhurt and ANP chief Asfandyar Wali Khan said his party would not be deterred by such terrorist activity.
In Balochistan, a boy was killed and five others were injured when a bomb went off near an election meeting by independent candidate Shams Mengal at Saryab Road in the provincial capital of Quetta.
Police said the bomb was planted in a bicycle that was left in the area. Police and paramilitary personnel took the injured to a nearby hospital. The banned Tehrik-e-Taliban claimed responsibility for the bomb attacks in Kohat and Peshawar. A Taliban spokesman said the group would continue targeting secular parties like the ANP, Pakistan Peoples Party and Muttahida Qaumi Movement.
Today's attacks came a day after six persons were killed and 65 others injured in three bombings that targeted MQM and PPP workers in Karachi. Most of the candidates contesting the May 11 general election from Pakistan's tribal belt have set up election offices in Khyber-Pakhtunkhwa.
Candidates are unable to campaign in the tribal areas because of threats from the Taliban and other militant groups. They have focussed on wooing voters in towns and villages located along the boundary with the seven semi-autonomous tribal agencies. The spate of attacks on election candidates and meetings has triggered concern about the security of political leaders and the polls.