The fresh violence comes days after ex-president Pervez Musharraf, a longtime U.S. ally, resigned as president, triggering a scramble for power that caused the country's ruling coalition to collapse.
The party long led by slain former Prime Minister Benazir Bhutto is now in a position to dominate the government and it is toughening its stance against Islamist extremists.
The Pakistani Taliban, meanwhile, are becoming increasingly bold, claiming responsibility for a wave of suicide bombings and gun attacks.
There was no immediate claim for Thursday's attack, though police said militants were the likely culprits. It happened as a van carrying prisoners crossed a bridge in the North West Frontier Province, said Waqas Ahmad, an area police chief.
The dead included police officers and prisoners, said Jalil Khan, another police official. But he could not provide an exact breakdown.
Hours earlier, security forces drove off a Taliban attack on a fort and pounded another band of militants holed up in a health center, officials said Wednesday as fighting spread to new areas in the tribal belt along the Afghan border.
As many as 49 insurgents were reported killed in separate attacks.
Pakistan's 5-month-old government initially sought to calm militant violence by holding peace talks.
But the initiatives have borne little fruit, and U.S. officials have been pressing for tougher action against insurgent groups blamed for rising violence across the border in Afghanistan and in cities further inland.
Musharraf quit Aug. 18, nine years after seizing power in a military coup, to avoid impeachment by political rivals who triumphed in February elections.
Former Prime Minister Nawaz Sharif pulled out of the coalition a week later over its failure to restore judges purged by Musharraf last year.
Lawyers who agitated for more than a year in favor of the judges and against Musharraf, restarted their rallies on Thursday.
Hundreds of demonstrators blocked roads in major cities. They also chanted slogans against Asif Ali Zardari, Bhutto's widower and the favorite to replace Musharraf when lawmakers select a new president on Sept. 6.
However, their numbers were smaller than in the past and Sharif's supporters largely stayed away.
Pakistan's army, which U.S. officials complain has been ineffective in preventing militants from mounting cross-border attacks into Afghanistan, is now fighting insurgents in at least three areas of the northwest.
Pakistani army chief Gen. Ashfaq Parvez Kayani met Adm. Mike Mullen, chairman of the U.S. Joint Chiefs of Staff, and other top American commanders Tuesday to discuss security strategy "in an open and cordial manner," a military statement said.
The gathering was prescheduled, it said, but gave no further details.
The New York Times reported that the meeting took place on a U.S. aircraft carrier in the Indian Ocean and focused on coordinating efforts against militants along the Pakistan-Afghan border.
Associated Press writers Zarar Khan in Islamabad, Ishtiaq Mehsud in Dera Ismail Khan and Ashraf Khan in Karachi contributed to this report.