The attacks were the deadliest to hit Bangladesh since the violence began in early January, when the opposition called a nationwide transport blockade.
Junior home minister Asaduzzaman Khan said transport operators had agreed to keep buses off the roads after 9pm, although other vehicles would still run.
"No night coaches will operate until further notice," he told reporters.
More than 1,000 buses, trucks and vans have been firebombed in attacks blamed on opposition activists as part of the blockade, aimed at toppling Prime Minister Sheikh Hasina.
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Millions of long-distance travellers rely on Bangladesh's buses, whose operators have suffered mounting losses since the blockade began.
Marcia Bernicat, the US ambassador, described the situation as "absolute tragic" after meeting the Bangladesh foreign minister A.H. Mahmood Ali today.
"Everyone has a role to play stopping that violence. Bangladesh is blessed with democracy and there is a space within that democracy to resolve differences," she said.
Opposition leader Khaleda Zia called for the current blockade of roads, railways and waterways after police confined the former two-time premier to her office on January 3.
Zia leads a 20-party opposition alliance which boycotted a general election last year on the grounds it would be rigged.
Authorities have deployed thousands of troops and police to guard vehicles and more than 10,000 protesters have been arrested, but the unrest shows no sign of abating.