Unidentified gunmen have shot dead a senior Sunni Muslim cleric in the Pakistani city of Karachi which has frequently seen sectarian clashes.
The cleric had a following in both Pakistan and Afghanistan
They ambushed the car of Mufti Nizamuddin Shamzai as he was being driven to the religious school where he preaches and three others were wounded.
Sunni Muslims, including his students, rioted at news of the death, attacking a police station and burning cars.
The governor of the surrounding Sindh region has appealed for peace.
One of the cleric's sons, a relative and the driver of their car were all injured in the attack which came as they were leaving the mufti's home near the Jamia mosque, police said.
"As soon as we sat in the car, we heard gunshots and we immediately ducked," the cleric's relative, named as Rafiuddin, told reporters.
"I felt a strong pain in my leg, I had been shot and then I saw Mufti Shamzai Sahib covered in blood."
Between four and six attackers escaped by motorbike and car, according to police.
Appeal for peace
It was not immediately clear if the motive for the attack had been sectarian but angry Sunni students from religious schools poured into the streets of this city of some 12 million.
Shamzai had been given armed guards after threats
"There is a lot of smoke in the air from the burning tyres and building, glass is scattered all around from damaged vehicles, and people are really charged," a Reuters correspondent reports from the scene.
Maj Gen Javed Zia, chief of the paramilitary rangers in the city, said his forces along with the police would defend Shia areas from attack.
"We fear a backlash on these areas," he said as reports came in that tear gas had been fired.
Ishrat-ul-Ibad Khan, governor of the Sindh region, vowed to bring the killers to justice, commending Mufti Shamzai for his "religious,
literary and social services".
Mufti Shamzai "always taught the lesson of peace, love and tolerance and today specially we need to follow the same - the only way to pay tribute to him", the governor was quoted as saying by AP news agency.
Mufti Shamzai was a strong critic of the US, urging a holy war after the invasions of Afghanistan and Iraq.
His religious school, known locally as Banuri Town, taught many members of Afghanistan's ousted Taleban movement.
Mr Shamzai himself had meetings with Taleban leaders on the eve of the US-led war to dislodge them.
The BBC's Zaffar Abbas reports from Islamabad that Mr Shamzai, 75, had been lying low since the ouster of the Taleban.
Karachi has recently seen an upsurge in sectarian violence. A suicide bomber killed at least 14 people at a Shia mosque earlier this month.