Police have accused Bangladesh's former prime minister and an opposition alliance leader of instigating a bus firebombing that killed seven people and injured 16 others.

Attackers threw the crude fire bombs at the packed bus as it traveled from the coastal city of Cox's Bazar to the capital, Dhaka, before dawn Tuesday during an opposition-sponsored strike.

Local police chief Uttam Chakrabarty said two cases were filed, accusing 56 named suspects and another 20 unnamed, while former Prime Minister Khaleda Zia has been named as the instigator.

Zia, her aides and opposition activists have denied involvement but did not immediately respond to the new allegations.

Another police official Nuruzzman Howlader said the prime accused suspect was a central leader of the Jamaat-e-Islami party from Comilla, the scene of the attack.