The explosion happened outside a public gathering organized by the Awami National Party in Charsadda, around 20 miles from Peshawar, a report by Pakistani newspaper Dawn said.
The bomb went off before the formal launch of the event and before ANP leaders, including chief Asfand Yar Wali Khan, arrived, the report said.
The local hospital confirmed seven people, including two policemen, were injured in the blast.
Security sources said that around 1 pound of explosive material was used in the bomb, which was detonated remotely.
Tehrik-i-Taliban Pakistan spokesman Ehsanullah Ehsan told Dawn in a telephone interview that the TTP claimed responsibility of the blast.
He warned people to stay away from all political gatherings, whether they are the ANP or the liberal Muttahida Quami Movement -- United National Movement.
The attack on the ANP gathering "is just a beginning," the spokesman reportedly said. "The attacks will intensify on both the secular ANP and MQM's political gatherings."
Ehsanullah also claimed responsibility for the fatal attack this week on a police station near Bannu, close to Pakistan's semi-autonomous tribal district of North Waziristan. At least eight people died including two suicide bombers, Dawn reported.
Officials in Bannu said three police officials were killed along with two civilians and an off-duty soldier. The victims were attending prayer services in the compound's mosque.
The militants were holed up in the mosque during the three-hour gun battle.
The TTP spokesman said two suicide bombers were sent to carry out the mission in Bannu.
He said the attack was retaliation for the recent killing of Ibrahim Mehsud, a nephew of former TTP chief Baitullah Mehsud who died in a U.S. drone strike in August 2009.
Baitullah Mehsud had a $5 million bounty on his head.
He was a member of the Mehsud tribe in Pakistan's South Waziristan region and increasingly had been active since the 9/11 attacks on the United States.
South Waziristan is regarded as a safe haven for al-Qaida and the Taliban but the Pakistani army has had some success in making it less safe in the neighboring Swat Valley.
Mehsud commanded up to 20,000 pro-Taliban fighters, mostly belonging to the Mehsud tribe, the BBC said. However, a report by Bloomberg News said he commanded a force of around 5,000.
Pakistan's security forces blamed Mehsud for the 2007 assassination of former Prime Minister Benazir Bhutto.