Parts of Karachi are on high alert following the attack
The head of Pakistan's largest extremist organization has been shot dead by armed militants in the southern province of Sindh, police say.
Maulana Ali Sher Hyderi, chief of the banned Sipah-e-Sahaba Pakistan (SSP) group, was travelling in his car in Khairpur district when it was attacked.
The SSP is an Islamic extremist organisation whose goal is to convert Pakistan into a "pure" Sunni state.
It is also said to have close links to the Lashkar-e-Jhangvi militant group.
The Lashkar-e-Jhangvi (LJ), is believed to have splintered from the SSP and is accused of being involved in hundreds of terror attacks across Pakistan.
"It was a targeted attack on Maulana Hyderi," the head of the local police in Khairpur district told the BBC.
"Both he and his driver were killed, while another man who was travelling with them was injured."
Maulana Hyderi was driving back to the southern city of Hyderabad, 200km (124 miles) east of the port city of Karachi, at the time of the attack.
According to police, he had just finished delivering a speech at an SSP public gathering.
As news of his killing spread, riots and demonstrations broke out across the country, especially in Karachi.
There were reports of aerial firing and shops being forced to close by armed activists.
Eastern Karachi remained particularly tense as vehicles were set on fire and attacked, before police and paramilitary troops were deployed to control the situation.
Businesses were closed down across other cities in the Sindh province, especially Hyderabad, Sukkur, Khairpur and Larkana.
Maulana Hyderi was the fourth chief of the SSP to be killed since it was formed in the late 1980s by Maulana Haq Nawaz Jhangvi.
Three were killed by armed assailants and one in a massive bomb attack in court premises.
The SSP, which was banned by President Pervez Musharraf in February 2002, has continued to operate under various names to counteract the ban.
After 2002, it changed its name to Millat-e-Islamia.
When this name was also placed on the banned list, the name of Ahle Sunnat Wal Jamaat was adopted.
The minority Shia community has often been targeted by the SSP
Although this is the official title, Sipah-e-Sahaba is the name everybody, including the party's own cadres, use to refer to the organization.
Since the 2002 ban, the party has revamped its structure and grown in strength and numbers.
While the organisation's leadership strongly denies this, security officials say the SSP has also strengthened its links with LJ, a group which remains Pakistan's deadliest terror outfit.
It has been accused of trying to assassinate at least two heads of state and of being involved in hundreds of other terror attacks across the country.
The LJ is also allied to the Baitullah Mehsud's Tehrik-e-Taliban Pakistan (TTP) organisation and maintains close links with al-Qaeda.
The SSP remains committed to increasing its political strength despite the clear ban against it, and has continued to hold rallies across the country.
Maulana Ali Sher Hyderi was a key figure in such gatherings as he was regarded as a capable and rousing orator.
Maulana Hyderi hailed from Khairpur and is expected to be buried there later on Monday amid tight security.