Pakistan's Dawn newspaper reported Friday at least eight people had died in Karachi and Lahore, the country's largest cities. Other reports put the death toll higher.
Bhutto, 54, a mother of three and the first female to become prime minister in the Islamic country, was killed Thursday at a political rally in the garrison city of Rawalpindi, near Islamabad. She had been campaigning to become prime minister the third time in Jan. 8 elections.
Dawn said paramilitary forces had been alerted to tighten security. Violence also was reported in Larkana, Bhutto's hometown near Karachi.
At least 150 vehicles, some gasoline stations and one hospital were set on fire in Karachi. The city descended into anarchy as armed mobs were joined by gangsters who held up panicked people stuck in traffic jams.
Telecommunications were jammed because of overloading.
In Lahore and elsewhere in the Punjab province, shops closed and vehicles stayed off roads, the report said.
In Peshawar, capital of extremist violence-wracked North-West Frontier Province, Bhutto's Pakistan People's Party supporters went on a rampage resulting in injuries to at least five people from police gunfire.