Dhaka - Hundreds of protesters have rallied in the Bangladeshi capital to denounce the murder of a prominent Bangladesh-born American blogger, who was hacked to death outside a university in Dhaka.

Students, activists and journalists gathered at Dhaka University on Friday to demand quick police action, a day after the attack on Avijit Roy and his wife Rafida Ahmed, who remains in critical condition.

The protesters said the killing raised questions about the government's ability to protect its citizens from crimes, and uphold democracy and freedom of speech.

In an interview with Al Jazeera, human rights activist Khushi Kabir questioned police inaction during the attack.

Witnesses have told Al Jazeera that police and onlookers were present during the attacks, but no one came to help the victims. Police were not immediately available to comment on the accusation.

The couple were coming from a book fair at the university, when a group of men ambushed them, with at least two of the attackers hitting them with meat cleavers, police Chief Sirajul Islam told the Associate Press news agency.

The attackers then ran away, disappearing into the crowds. Two blood-stained cleavers were found after the attack, he said.

On Friday, Al Jazeera learned from the family of the slain victim that his body will be donated to the Dhaka Medical College Hospital (DMCH) for medical research.

Avijit's younger brother Arijit Roy told journalists that family members have taken the decision to donate the body, according to the wishes of the victim.

'Shocked and heartbroken'

Protesters said the attack on Roy and his wife happened partly because of the government's failure to prosecute previous deadly attacks.

"The government cannot avoid its responsibility as it has failed to try similar crimes before," University professor Anwar Hossain told Al Jazeera.

In 2013, another blogger, Ahmed Rajib Haider, who also spoke out against religious fanatics, was killed by unidentified assailants near his home in Dhaka.

Activisit Jonayed Saki demanded that police immediately arrest the suspects.

"How can such attacks ensue amid tight security," he asked.

The Centre for Inquiry, a US-based nonprofit group Roy wrote for, said it was "shocked and heartbroken" by the murder.

"Dr Roy was a true ally, a courageous and eloquent defender of reason, science and free expression, in a country where those values have been under heavy attack," it said in a statement.

As night fell in Dhaka on Friday, student and activists held a torch procession to protest Roy's death.