Geo News, quoting police, said the Tuesday attack in the Yakatoot area of the city in northwest Pakistan was carried out by a suicide bomber and at least 17 people including policemen died and another 60 were wounded.
There were up to 300 party workers and leaders including Ghulam Ahmed Bilour and his nephew Haroon Bilour at the meeting, the report said. Both the Bilours escaped the bombing.
The Tehrik-e-Taliban Pakistan claimed responsibility, Geo News said.
The TTP has warned Pakistanis to stay away from the elections.
It was the fourth such act of deadly violence as Pakistanis prepare to hold their general elections May 11.
Pakistan's secular parties such as the ANP and the Muttahida Qaumi Movement are under threats from militants ahead of the elections.
"A suicide bomber, who was standing among the workers waiting outside the meeting venue to welcome the leaders, jumped in front of Bilours car and blew himself up," a police official told Geo News.
The report said several vehicles parked nearby caught fire after the bombing.
The injured were taken to area hospitals.
Pakistan's Express Tribune said the latest attack has once again raised concerns about security of election candidates.
Some of the candidates criticized the government and the elections commission for inadequate security.
Asfand Yar Wali, a leader of the ANP, told reporters it was the fourth attack on the party's leadership in the past week.
"We become an easy target of militants as we couldn't get proper security despite our repeated request to the government," said Wali, Xinhua News Agency reported.
On Sunday, Mukaram Shah, a local ANP leader, was killed in an explosion, which was also claimed by the TTP, Dawn reported.
Both Pakistani President Asif Ali Zardari and caretaker Prime Minister Mir Hazar khan Khoso condemned the incident.
The civilian coalition government, led by Zardari's Pakistan People's Party, for the first time in Pakistan's history completed its full five-year term last month. The May 11 elections would be an important political milestone as they would mark the first such democratic power transition in a country that has been intermittently under military rule for decades since independence in 1947.
The TTP, which is separate from the Taliban in Afghanistan, is close to al-Qaida.
Its leader, Hakimullah Mehsud, in a recent video, said the elections are intended to divide Muslims, CNN reported. Mehsud said his group wants "the implementation of Sharia [law] and for that jihad is necessary."