PESHAWAR, Pakistan — A suicide bomber wearing 20 pounds of explosives assassinated the prominent leader of a Pakistani security force and three other people on Wednesday afternoon at a busy intersection here in northwest Pakistan, security officials said.

A spokesman for the Pakistani Taliban claimed responsibility for the attack against the principal victim, Safwat Ghayur, the commander of Pakistan’s Frontier Constabulary, a force controlled by the Interior Ministry that has carried out campaigns against militants in Pakistan’s semiautonomous tribal areas.

A car carrying Mr. Ghayur, 50, had just left the force’s headquarters and was stopped at a traffic signal when a young suicide bomber ran up and detonated his explosives, Mr. Ghayur’s driver told journalists at the city’s main hospital.

The driver, Shakirullah Khan, who suffered minor injuries, said he ran to the passenger side to try to rescue Mr. Ghayur, but “the car was engulfed in flames and I couldn’t save him.”

Shafqat Malik, leader of the bomb disposal squad, said the bomber had been carrying 20 pounds of explosives loaded with ball bearings.

Eleven people were wounded in the attack, which came as residents in much of northwestern Pakistan were struggling to recover from severe floods that killed at least 800 people, displaced as many as 1.4 million and devastated livestock herds.

Azam Tariq, the spokesman for the Pakistani Taliban, which is formally known as Tehrik-i-Taliban Pakistan, told the newspaper Dawn that militants would continue to strike at law enforcement agencies and leaders of the Pashtun-dominated provincial governing party, the Awami National Party.

“They are on our hit list,” Mr. Tariq said in a telephone interview from an undisclosed location. “We shall show no leniency.”

Militants have aimed attacks at police and security officials here in Pakistan’s most restive province and throughout the country as the Pakistani military has waged a campaign against militant groups in tribal areas along the border with Afghanistan.

Mr. Ghayur was described as a widely respected and bold police officer and was credited with many of the successes in the war against Pakistani militants.

“He was one of the best officers,” said Pakistan’s interior minister, Rehman Malik. “We all mourn his death.”

One witness, Hameed Khan, told reporters that he and a friend had been delivering some relief supplies for flood victims when the blast struck. Their car was burned, but they were unharmed, he said.

The government of the province of Khyber-Pakhtunkhwa, formerly the North-West Frontier Province, announced a day of mourning on Thursday. Flags will be flown at half-staff.