(Adds detail, higher toll)
By Imtiaz Shah
KARACHI, Dec 28 (Reuters) - Troops were called out on Friday to quell some of Pakistan's worst political violence in years, sparked by the assassination of opposition leader Benazir Bhutto.
Officials said 31 people, including four policemen, had been killed since the former prime minister was murdered on Thursday in a bomb attack after an election rally.
Most of the dead were killed in Sindh in the south, Bhutto's home province and main power base.
Troops were deployed in several parts of Sindh, officials said, and banks and schools were closed across the country.
In the city of Hyderabad, police and witnesses said protesters had set fire to about 25 banks, to 100 vehicles and to foreign fast-food outlets, despite orders to the police and paramilitary forces to shoot violent protesters on sight.
Several train coaches were also torched.
Interior Ministry spokesman Javed Iqbal Cheema told a news conference that "vested groups" and criminals were taking advantage of the situation to loot and rob banks.
Officials had said they feared the disturbances would intensify after Bhutto's funeral at her family's ancestral home in the province on Friday afternoon.
Meanwhile, in a suspected Islamic militant attack in the northwesterly Swat valley, where the army has been fighting pro-Taliban forces, a blast at an election meeting of the Pakistan Muslim League (Q) party killed six people including an election candidate, police said. The party backs President Pervez Musharraf.
Islamist militants, probably linked to al Qaeda, top the list of suspects for Bhutto's murder, and the Interior Ministry said it had "intelligence intercepts" indicating that al Qaeda was behind the killing.
KARACHI'S STREETS DESERTED
In Karachi, Pakistan's biggest city and its commercial capital, more than 2,000 people set a police station on fire and torched cars and stole weapons, police said.
In the east of the city, at least six people died in a factory set alight by protesters. Elsewhere, streets were largely deserted, with shops shuttered.
"Since last night a lot of damage has been caused. Shops, cars and government buildings are being burnt," said senior Karachi police official Azhar Ali Farooqi.
Fires also blazed across the interior of Sindh.
A Reuters reporter travelling from Karachi to Bhutto's home district of Larkana, where she was buried, saw hundreds of smouldering shops and vehicles. Many protesters shouted slogans against Musharraf, Bhutto's old rival.
In the eastern city of Lahore, one person was killed in protests, police said.
In Peshawar, in the northwest, two offices belonging to pro-Musharraf political parties were torched, a witness said.
In the southwesterly province of Baluchistan, protesters set fire to a railway station, several banks, government vehicles and the offices of a pro-Musharraf party.
In the city of Multan in Punjab province, a crowd damaged seven banks and torched eight petrol stations.
Protesters also took to the streets of Islamabad and Rawalpindi, smashing windows and setting fire to tyres. (Additional reporting by Asim Tanveer; writing by Robert Birsel; Editing by Kevin Liffey)