Bangladeshi opposition parties have called a two-day nationwide shutdown from Wednesday to protest what they describe as the "mass killing" of Islamists in a crackdown by security forces.

The main opposition Bangladesh Nationalist Party and its Islamist allies called the strike after saying that hundreds of people died on Sunday and early Monday, when police broke up a mass rally in central Dhaka.

According to AFP, 38 people are known to have been killed since Sunday afternoon when police first confronted Islamist activists who had blockaded the capital.

The Islamists are trying to pressure the government into introducing a new blasphemy law, and have been calling for the execution of bloggers they accuse of insulting the Prophet Muhammad.

A border guard died from head injuries on Tuesday to raise the toll from Monday's 37, police inspector Mozammel Haq said, adding that dozens are still in hospital.

The country's most prominent daily Prothom Alo said at least 49 people have died in the clashes, some of the fiercest street violence in decades.

"We have called two days of nationwide strike to protest the mass killing of Hefajat-e-Islam workers and supporters on Sunday and Monday," BNP spokesman Khandaker Mosharraf said on Tuesday.

Police on Tuesday announced charges against 194 activists of the Hefajat-e-Islam (Protectorate of Islam), a hardline Islamic group behind the protests.

Its secretary general, Junayed Babu Nagori, who was detained on Monday night, faces a murder charge, police sub-inspector Tabibur Rahamn said.

Hefajat's main leader, 90-year-old Allama Ahmad Shafi, was put on a plane to the country's second city Chittagong on Monday where his supporters clashed with police, leaving at least five people dead.

Information Minister Hasanul Haque Inu accused the heads of religious seminaries of encouraging "terrorist activities" by sending their students out to join the protests. "The madrassa superintendents who are encouraging their students to take part in terrorist activities will be tried," he said.

Chanting "Atheists must be hanged", Hefajat activists marched along at least six highways on Sunday, effectively cutting Dhaka off from the rest of the country. Police said the number of protesters reached around 200,000 at one point.

The protest by Hefajat, which has drawn support from madrassas, is another sign of the divide between Islamists and the secular government, after the deaths of about 100 people earlier this year in violence linked to war crimes trials.

Three leading Islamists have so far been convicted by a special tribunal for their role in mass killings during the 1971 independence war, which saw what was then East Pakistan break away to become Bangladesh.

The overall death toll in violence between religious hardliners and the police since January stands at about 150.

UN Secretary-General Ban Ki-moon has called for an end to the violence, expressing his sadness at the loss of life.

Ban "urges all concerned to stop the violence, to respect the law and to express their views peacefully," said a statement issued by his spokesman.

AFP-Xinhua