Attackers threw crude fire bombs at a packed bus in Bangladesh early Tuesday, setting it alight as it moved along a highway and leaving at least seven people dead and 16 injured during a nationwide strike called by the opposition.

The pre-dawn attack, the latest during a surge in political violence, came as the bus traveled from the coastal city of Cox's Bazar to the capital, Dhaka, said Uttam Chakrabarty, police chief in the Comilla district where the bombing occurred. The area is about 90 kilometers (55 miles) east of the capital.

He blamed opposition activists for the attack, but they denied involvement.

The injured have been hospitalized, mostly for burns, he said.

"I along with my cousin opened the window and got out," said Muhammad Shariful, a 19-year-old survivor. "Later we noticed that my friend had not come out. We searched the surrounding area, and then saw that he was lying at one side of the street with fire burning all over his body."

Political attacks — mostly fire bombings of vehicles — have killed at least 53 people since early January, when an opposition alliance led by former Prime Minister Khaleda Zia ordered a nationwide transportation blockade in an attempt to force Prime Minister Sheikh Hasina to step down. The two politicians have long battled for political supremacy in the South Asian nation.

The bus attacked Tuesday, unlike many vehicles traveling on major roads since the blockade was ordered, had no security escort from the police or the country's paramilitary border guards.

Authorities blame opposition activists and hired thugs for the attacks. Zia and her aides deny involvement.

Tuesday was also the last day of a 72-hour nationwide general strike ordered by the opposition, which called for schools, offices and businesses to close.

Zia's party and its partners boycotted 2014 elections after being told there would be no neutral monitor overseeing the voting. That allowed Hasina to win a new 5-year term. Hasina says new elections will not be held until 2019.

The renewed violence ended a year of relative calm in Bangladesh, where politics has long been accompanied by chaos. Political violence left nearly 300 people dead in 2013.

Zia was prime minister from 2001 to 2006, but failed to hand over power peacefully. A military-backed caretaker government ruled the country for two years before Hasina came to power in 2008 elections.