Dhaka: Bangladesh vowed Friday to catch the killers of a student murdered this week after criticising Islamists on social media, as hundreds of secular activists held a protest to demand action.

Nazimuddin Samad, a 26-year-old law student, was killed late on Wednesday near his university in Dhaka by unknown assailants carrying machetes.

It was the latest in a series of murders of secular bloggers and campaigners in Bangladesh and has sparked international outrage, as well as demands for the government to protect freedom of speech in the Muslim-majority country.

Police on Friday filed a murder case and said they were treating his death as a "targeted killing", although no arrests have yet been made.

Abu Hena Muneem, a senior home ministry official, dismissed claims the government was failing to protect secularists and said the authorities were doing all they could to track down Samad's killers.

"The accusations are not correct. Our law enforcement agencies are working very hard to find the culprits and they will soon be arrested," Muneem told AFP.

Activists, however, expressed concerns about the government's readiness to protect them as they held a protest march in the capital.

Around 400 people chanted slogans including "stop the culture of impunity, save secular Bangladesh".

"It is very worrying," said Imran Sarker, a spokesman for Bangladesh's biggest secular activists' group Gonojagoron Mancha.

"We wonder whether the government actually has the goodwill to put an end to this."

Samad's murder was the sixth such killing in 15 months.

His childhood friend and fellow activist Gulam Rabbi Chowdhury said he had gone into hiding before the attack and deactivated his Facebook page for a number of months.