DHAKA, Bangladesh — A Bangladeshi-American blogger known for his antipathy to religion was hacked to death on the street in this capital city by two assailants wielding machetes, the police said on Friday.

The victim, Avijit Roy, 42, was leaving a book fair with his wife Thursday evening when his attackers approached him from behind, according to the police. His wife, Rafida Ahmed Bonya, 45, suffered a blow to the head and was in critical condition in a Dhaka hospital, said Sirajul Islam, an officer at the Shahbag police station, where Mr. Roy’s father reported the assault.

Jibon Ahmed, a photographer for a local photo agency, said by phone that he heard screams outside the fair around 9 p.m. After finding the couple, he said, he helped them into an auto-rickshaw and took them to the hospital, where Mr. Roy died.

The police have not named any suspects, but Mr. Islam said that witnesses had provided descriptions of the attackers. Officers recovered two bloodied machetes from the scene of the crime.