At least 27 people were killed and scores injured in clashes in Bangladesh on Sunday, as police fired tear gas and lobbed stun grenades to disperse tens of thousands of protesters calling for Prime Minister Sheikh Hasina to resign.

The interior ministry declared an indefinite nationwide curfew starting at 6:00 pm (1200 GMT) on Sunday, the first time it has taken such a step during the current protests that began last month.

The unrest, which has prompted the government to shut down internet services, is its biggest test since deadly protests erupted after Hasina won a fourth straight term in January elections that were boycotted by the main opposition Bangladesh Nationalist Party.

Critics of Hasina, along with human rights groups, have accused her government of using excessive force to stamp out the movement, a charge she and her ministers deny.

Demonstrators blocked major highways on Sunday as student protesters launched a non-cooperation program to press for the government's resignation, and violence spread nationwide.

"Those who are protesting on the streets right now are not students, but terrorists who are out to destabilise the nation," Hasina said after a national security panel meeting.

"I appeal to our countrymen to suppress these terrorists with a strong hand."

Two construction workers were killed on their way to work and 30 injured in the central district of Munsiganj, during a three-way clash of protesters, police and ruling party activists, witnesses said.

"They were brought dead to the hospital with bullet wounds," said Abu Hena Mohammad Jamal, the superintendent of the district hospital.

CLASHES

Police said they had not fired any bullets, however, when some improvised explosives were detonated and the area turned into a battleground.

In the northeastern district of Pabna, at least three people were killed and 50 injured during a clash between protesters and activists of Hasina's ruling Awami League, witnesses said.

Two more were killed in violence in the northern district of Bogura, and 20 were killed in nine other districts, hospital officials said.

"An attack on a hospital is unacceptable," said Health Minister Samanta Lal Sen after a group vandalised a medical college hospital and set fire to vehicles, including an ambulance, in Dhaka, the capital. "Everyone should refrain from this."

For the second time during the recent protests, the government shut down high-speed internet services, mobile operators said, while social media platforms Facebook and WhatsApp were not available, even via broadband connections.

Last month, at least 150 people were killed, thousands injured and about 10,000 arrested in violence touched off by demonstrations led by student groups protesting against quotas for government jobs.

The protests paused after the Supreme Court scrapped most quotas, but students returned to the streets in sporadic protests last week, demanding justice for the families of those killed.