Masked gunmen killed a supporter of Benazir Bhutto early today, while security forces shot dead two other party activists as a mob tried to force its way into an oilfield, police said.
The killings take the death toll since Bhutto's assassination on Thursday to 40, including four policemen, and came as protesters torched shops, lorries, welfare centres and ambulances overnight as violence entered a third day.
A 27-year-old man wearing a tunic made from a Pakistan People's Party (PPP) flag had just shouted "Bhutto is great" when he was gunned down while returning from the mausoleum where Bhutto was buried on Friday, police said.
"Two gunmen were waiting in a vehicle, their faces covered, and they opened fire," said Shaukat Ali Shah, deputy inspector general of police in the city of Hyderabad in Sindh.
Separately, up to 400 PPP activists carrying banners portraits of Bhutto and wielding bricks, tried to burst into an oilfield facility near Hyderabad before dawn, when security forces acted on orders to shoot violent protesters on sight.
"The mob was warned," Shah said. "Two people were killed."
Almost all of the deaths since Bhutto's killing occurred in the southern province of Sindh, the PPP's power base, where the Election Commission said several of its offices were set on fire and electoral rolls and ballot boxes destroyed.
Downtown neighbourhoods of Karachi, the volatile capital of Sindh, and a city of 14 million people, were virtually deserted, as armed police and paramilitary forces patrolled streets. Shops and petrol pumps were closed as the city observed a national three-day mourning period.
There were isolated acts of violence and arson, and roads were littered with broken glass and charred shells of cars.
There was firing in the air in Lyari, a neighbourhood of Karachi where support for Bhutto runs deep, according to a paramilitary spokesman. Buildings and ambulances run by the Edhi Foundation, a charity that runs welfare centres, were vandalised.
Telephone and internet links between Islamabad and Karachi were also disrupted due to the violence. The main fibre-optic between the two cities was cut in Sindh province and an angry mob on Saturday chased away engineers sent to fix it.
Sindh home secretary Ghulam Mohtaram said rioters had burned or damaged 947 vehicles, 131 banks and 31 petrol stations up until midnight on Friday. Dozens of banks, vehicles and fast food outlets were set on fire in Hyderabad.