At least 15 people were killed and dozens injured in a suspected suicide blast targeting Shias in the southern Pakistani city of Jacobabad Friday, police and hospital officials said.

The attack came with Pakistan on high alert for the mourning days of Ashura, during the holy month of Moharram, a flashpoint for sectarian violence in previous years.

"Many injured and bodies were brought to the Civil Hospital," senior police officer Zafar Iqbal told AFP, who initially described it as a suicide attack but later said authorities were still investigating.

Civil Hospital chief Dr Altaf Wagan confirmed that at least 15 bodies and more than 36 injured people had been brought in, and said that more wounded had been shifted to two other hospitals nearby.

The attack came outside the residence of a local Shia leader as devotees were setting off towards a main procession in the city, some 500 kilometres (300 miles) from Karachi, Iqbal said.

Witnesses described the wounded being rushed to hospital by ambulance and auto-rickshaw. Reports said there were children among the injured.

"We were some three kilometres (two miles) from the spot and heard the blast," Jan Odhano, a rights activist in Jacobabad, told AFP.

"We rushed towards the spot. We saw people running here and there, some were crying and wailing, we could see blood on the clothes of some people."

Provincial transport minister Mumtaz Jakharani, who was at the hospital, said that 15 to 20 people were "martyred" in the attack.

He said protesters had gathered at the hospital, with some damaging hospital equipment.

"The doctors are scared of the agitating people," provincial health minister Jam Mehtab Dahar told AFP, confirming that at least 15 people had been killed.

"I call upon the protesters to calm down and help us treat the injured."

Witnesses said protesters were blocking roads at many spots in the city.

Dahar confirmed that the injured were also sent to the hospital at the Shahbaz Airbase and the Jacobad Institute of Medical Sciences, with some airlifted to the nearby city of Larkana.

Pakistan had deployed some 10,000 troops and 6,000 paramilitary members to prevent sectarian violence during Moharram.

The holy month, which began Thursday, sees Shias hold processions and gatherings to mourn the martyrdom of the Prophet Mohammed's grandson Imam Hussain.

Friday's attack came a day after a suicide bomber blew himself up at a mosque, killing at least 11 Shias including six children in the town of Chalgari in restive Baluchistan province.

Clashes between Sunnis and Shias led to at least 11 deaths two years ago in the garrison city of Rawalpindi close to the capital Islamabad.

Sectarian violence -- in particular by Sunni hardliners against the Shias that make up roughly 20 percent of Pakistan's 200 million people -- has claimed thousands of lives in the country over the past decade.