A Bangladeshi law student who criticised Islamism on his Facebook account has been attacked with machetes and killed, police said, the latest in a series of murders of secular activists and bloggers by suspected Islamist militants.

Secular blogger Nazimuddin Samad, 28, was attacked as he was returning from a law class at his university in the capital, Dhaka, late on Wednesday, police said.

Last year, suspected militants killed five secular writers and a publisher, including a Bangladeshi-American activist. A banned Islamist militant group, Ansarullah Bangla Team, claimed responsibility for some of the attacks.

"At least four assailants hacked Nazimuddin Samad's head with a machete on Wednesday night. As he fell down, one of them shot him with a pistol from close range. He died on the spot," Syed Nurul Islam, deputy comissioner of Dhaka Metropolitan police, told AFP.

"It was a case of targeted killing. But no group has claimed responsibility," Islam said.

Witnesses heard the attackers shouting "Allahu Akbar" (God is Greatest) as they fled, police officer in charge of the Sutrapur area of Dhaka, Tapan Chandra Shaha, told Reuters.

"Nazim was both hacked and shot. We have recovered bullet shells from the spot. He has been hacked on the right side of head," Shaha told the Guardian.

Imran H Sarker, convener of the BOAN online activist group, said Samad was an outspoken critic of injustice and militancy.

"We found him always a loud voice against all injustice and also a great supporter of secularism," said Sarker.

On his Facebook page, Samad described his views on religion, stating: "Evolution is a scientific truth. Religion and race are invention of the savage and uncivil people."

Bangladesh has seen a wave of militant violence over the past year or so, including a series of bomb attacks on mosques and Hindu temples.

Some recent attacks have been claimed by Islamic State, including the killing of Hindu priest, a Japanese citizen, an Italian aid worker and a policeman.

The government denies that Islamic State has a presence in the Muslim-majority country of 160 million people.

Hundreds of students from the Jagannath University where Samad studied protested against his murder and demanded the prompt arrest of the killers.

They blocked roads in and around the university and told reporters that if those behind the earlier murders of bloggers had been punished then Samad would not have been attacked.

Additional reporting by Reuters