Police say unidentified attackers have hacked a prominent US blogger to death in Bangladesh's capital, Dhaka.

Police on Friday said that Avijit Roy, a Bangladesh-born US citizen, known for this writing against religious fundamentalism, and wife Rafida Ahmed, were attacked after a visit to a book fair at Dhaka University.

Ahmed, also a blogger, sustained severe injuries in Thursday's attack. 

Sirajul Islam, a local police chief, told The Associated Press news agency the assailants used cleavers to attack the couple.

"Several attackers took part in the attack and at least two assailants hit them directly,'' Islam said, adding that two blood-stained cleavers were found after the attack.

Al Jazeera's Tanvir Chowdhury, reporting from Dhaka, said it was not known who was behind the attack, but the family of the victims are blaming "religious fanatics."

Witnesses told our correspondent that police and onlookers were present during the attack, but no one came to help the victims. 

On Friday, dozens of people protested at Dhaka University, near the site of the attack, calling for justice for Roy and his wife. 

 Previous attacks have been blamed on religious fanatics [AFP]

Roy had founded a popular Bengali-language blog - Mukto-mona, or Free Mind - in which articles on scientific reasoning and religious extremism featured prominently.

Anujit Roy, his younger brother, says Roy had returned to the country earlier this month and was planning to return there next month.

Similar attacks have previously taken place in Bangladesh.  

Investigators have said religious fanatics were behind those attacks. In 2013, another blogger, Ahmed Rajib Haider, who also spoke out against religious fanatics, was killed by unidentified assailants near his home in Dhaka.

Source: Agencies