Anti-government protesters firebombed a bus full of sleeping passengers in eastern Bangladesh on Tuesday, killing seven in spiralling political unrest aimed at toppling Prime Minister Sheikh Hasina.
Several passengers were also critically injured in the attack in Chuddogram town blamed on activists from the main opposition Bangladesh Nationalist Party (BNP) led by two-time former premier Khaleda Zia.
The deaths brought the number of people killed in the month-long protests to 53 -- most of them victims of firebomb attacks on buses and lorries -- as opposition activists try to enforce a transport blockade.  
"Seven passengers were burnt to death in the bus after the petrol bomb was thrown at 4am (2000 GMT Monday)," district police chief Tuttul Chakrabarty told AFP by phone.
"Five of the passengers are fighting for their life as 40-80 percent of their bodies were burnt. They have been shifted to a hospital in the capital," he said.
Authorities have stepped up the pressure on 69-year-old Zia, who has been holed up in her office since January 3, in a bid to halt the violence.
Zia called the protests early last month, urging supporters to enforce a nationwide blockade of roads, railways and waterways to force Hasina to call a fresh election.
The BNP and its allies boycotted the last poll in January 2014 on the grounds that they believed the result would be rigged.
Authorities on Monday ordered a probe into allegations of murder against Zia over the deadly protests, while one of her closest aides, a media magnate, was arrested.
- 'People burning in bus' -
Survivors of Tuesday's attack said the bus was packed with local tourists returning overnight from the resort town of Cox's Bazaar to the capital Dhaka.
"I woke up hearing loud cries and saw people burning in the bus. I jumped through the window and found a friend in flames. I doused the fire but his condition is critical," one survivor told private Ekattur TV.         
Up to 15 people were also slightly injured after they jumped from the vehicle's windows, trying to escape the blaze, the police chief added.
Police inspector Mahfuzur Rahman told AFP authorities suspected BNP activists were behind the petrol bomb. Raids were carried out on local villages near the location of the attack, but no arrests were made.
In another attack on Tuesday, four people suffered burn injuries after a petrol bomb was thrown at a moving train outside Dhaka, police said.  
Hundreds of people have been injured since the protests started, while local media say more than 850 vehicles have been torched or damaged and inter-city transport services have ground to a halt.
Security forces have launched a nationwide crackdown -- but the arrest of more than 10,000 opposition supporters appears to have done little to quell the unrest.
Zia denies her BNP and its Islamist allies are responsible for the firebombings and have demanded the release of opposition officials and leaders detained over the violence.
Hasina has accused her bitter rival Zia of trying to trigger "anarchy", and authorities ordered the probe on Monday against the opposition leader into allegations of murder made by a pro-government activist.
Zia was charged last week over lesser crimes of "abetting" and "instigating" the firebombings.
Police also arrested on Monday one of Zia's key aides, Mosaddek Ali Falu, the owner of the country's leading television station, over an arson attack in central Dhaka.
Western countries including the EU, the impoverished nation's biggest export destination, has urged Hasina's government and the opposition to hold talks to resolve the crisis.
Source: AFP