A bomb attack killed at least 11 police officers and wounded 42 others in Pakistan's southern city of Karachi on Thursday, police said.
A van exploded after hitting a bus the officers boarded moments after it had left a training school in what appeared to be a suicide attack, said police officer Rao Anwaar.
Nearly 50 officers on board the bus came from a special commando force, he said.
Pakistani TV channels showed the blast scene littered with broken glass and pieces of vehicles scattered around.
The wounded officers were rushed to hospitals, and 12 of them were in critical condition, said Dr Seemi Jamali at the city's Jinnah Hospital.
No one has claimed responsibility for the attack in Pakistan's business hub. The port city of Karachi has long been a center of ethnic, political, militant and sectarian violence.
The assault is latest in a series of attacks at a time when Pakistani government is trying to strike a peace deal with local Taliban militants fighting in the country's northwest to end the violence that has killed thousands of security forces personnel, government officials, political workers and civilians in recent years.
Militants killed nine members of an anti-Taliban militia on Wednesday in Pakistan's northwestern city of Peshawar, a day after assailants threw hand grenades inside a cinema, killing 13 in the city.
Peshawar sits near restive areas along the Afghan border that are home to Pakistani Taliban and al-Qaida-linked foreign Islamic militants.
The Pakistani Taliban have been waging a bloody war against the government in a bid to overthrow the authorities and enforce their harsh brand of Islamic Shariah law.
The Pakistani Taliban, formally called Tehreek-e-Taliban Pakistan, are separate from the Taliban fighting Nato-forces in neighboring Afghanistan. Although the two groups share similar ideology, the Pakistani Taliban have focused their fight against the Pakistani government.
Pakistani Prime Minister Nawaz Sharif has made negotiating with the militants a centerpiece of his new government elected last May.
After some initial stumbles, the government's efforts have picked up steam in recent weeks with both sides naming people to represent them in the talks. Members of the Pakistani Taliban's negotiating team flew to the North Waziristan tribal agency over the weekend to meet with the militant organisation's leadership at a secret location.
Maulana Samiul Haq, who heads the Taliban's negotiating team, said both the militants and the government have recommended a cease-fire as a confidence-building measure.
The two teams are scheduled to meet again Thursday.
Critics say the militants have used such peace deals to gain time to strengthen themselves and regroup.