PESHAWAR, Pakistan  A huge and lethal blast rocked a crowded market in the northwestern city of Peshawar on Friday, in what appeared to be a warning about the government’s plans to launch a military offensive against militants in the frontier region of South Waziristan.

The blast, which police and security officials suspected was caused by a suicide car bomb containing more than 100 pounds of explosives, was the biggest in Pakistan in months, killing at least 48 people, including seven children and one woman, and wounding 148 others.

It was the second attack by militants this week, after the bombing of a United Nations agency on Monday, raising concern that Taliban militants were preparing a new wave of attacks in a country where scores of suicide bombings have occurred.

Peshawar, the capital of the North-West Frontier Province, has long been an easy target for the militants; it is also crucial to both the Taliban and the government because of its proximity to Pakistan’s mountainous frontier. Furthermore, the city is of strategic value to NATO because it serves as a transportation hub for supplies bound for neighboring Afghanistan.

Image The aftermath of the explosion in Peshawar on Friday. Credit... Reuters

A majority of the people killed were passengers traveling in a public minibus, which was passing beside the car used in the attack. Several pedestrians were also killed or seriously wounded.

The blast was so intense that it overturned the passenger bus, leaving bodies and injured passengers trapped in its wreckage. The bombing also damaged dozens of multistory buildings, shops and offices in the area, a known commercial center, and snapped the power supply cables. Witnesses said many of the dead were damaged beyond recognition.

“Out of the 48 bodies, so far 27 have been identified,” said Dr. Sahib Gul, chief of the trauma center at Lady Reading Hospital, where most of the casualties were taken. He feared that the death toll might increase, as some of the wounded were in critical condition.

Officials said the militants, who largely attack military or police personnel, were expanding their targets to include civilians to press the government and forestall a possible operation in the South Waziristan tribal region.

The Provincial Assembly was in session about a half-mile away, and local television coverage showed legislators emerging from the building and making calls on their cellphones.

The United States ambassador, Anne W. Patterson, immediately condemned the attack, saying in a statement that it “serves only to highlight the vicious and inhuman nature of the terrorists who aim to instill fear in the hearts of the Pakistani people.”

Image A man carried an injured woman after a suicide bombing in Peshawar, Pakistan, on Friday. Credit... Mohammad Sajjad/Associated Press

Meanwhile, Rehman Malik, the Pakistani interior minister, said Friday that investigators had made one arrest and identified those involved in the suicide bombing of the headquarters of the World Food Program in Islamabad, the capital, on Monday. Four Pakistanis and an Iraqi died in the blast.

Speaking to reporters outside the Parliament in Islamabad, Mr. Malik said the government knew who took the bomber to the highly fortified building of the food program.

“The main handler is in our custody, and we have complete information about the persons involved in executing the terrorist plan and facilitating the terrorists,” Mr. Malik said. “We know where the instructions came from, where he stayed.”

The Taliban claimed responsibility for the attack, and a Taliban spokesman, Azam Tariq, said there would be more singling out of foreigners and Pakistanis working with overseas agencies.

“Given this aggressiveness and this new threat, we are taking a range of actions to redouble our security efforts,” said Josette Sheeran, the executive director of the food program, which says it supplies food and humanitarian aid to an estimated 10 million Pakistanis.

The program is a United Nations agency, and the organization has temporarily closed its offices in Pakistan. Ms. Sheeran said the withdrawal of World Food Program staff members from the country was “not contemplated.”