Police said suspected activists of former prime minister Khaleda Zia's Bangladesh Nationalist Party (BNP) hurled petrol bombs on the bus at around 3.30 am as it was heading towards the capital from southeastern sea resort town of Cox's Bazar.
"The bus came under attack at Chouddogram area (on the Dhaka-Chittagong highway) and we sent the seriously injured passengers to the DMCH (Dhaka Medical College Hospital) burn unit and others to (nearby) Comilla Medial College Hospital," an official of highway police told newsmen.
Doctors at the facility here said the toll could rise as burn injury of some of the wounded passengers appeared beyond treatment while three of them sustained "70 per cent burns".
In two near simultaneous assaults, suspected opposition activists uprooted railway tracks causing derailment of a train near southeastern port city of Chittagong and threw petrol bombs on another train in the outskirts of the capital Dhaka leaving 14 passengers injured in the two incidents.
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This was the second such attack on trains in 24 hours after saboteurs uprooted tracks derailing a train, injuring five people and delinking Chittagong with rest of the country for hours in railway routes.
In the last 24 hours, at least 15 vehicles came under arson attacks, five of those in the capital injuring several as the BNP-enforced 72-hour strike is underway.
In another incident, two motor-bikers were killed after being hit by a truck when they tried to set it on fire in western Monirampur of Jessore district.
BNP visibly intensified its violent campaign after a Dhaka court yesterday ordered police investigations against Zia on a complaint accusing her of masterminding deaths of at least 42 people by spearheading the unrest. This was the fourth such case against Zia within a week.
Prime Minister Sheikh Hasina warned Zia, saying she would have to "suffer a lot" for spearheading the violent campaign to topple her government at the cost of innocent lives.
"She (Zia) will suffer much. We can take many steps against her but I don't want to do anything right now," a ruling Awami League lawmaker quoted the premier as saying at a meeting of their parliamentary party last night.