The device was in a package sent to the office of the Liberal Islam Network, a grouping of religious intellectuals, in East Jakarta, national police spokesman Boy Rafli Amar told reporters.
"We suspected that it was a bomb. Our officers were trying to tame it when it went off," he said, adding that the police are investigating the motive.
A witness told local television station MetroTV that the package was addressed to the group's former director and liberal Muslim scholar Ulil Abshar Abdalla.
"Our office received a suspicious package containing cables with a strong smell. We called the police," Fia Anwar said.
"As the police were checking the package, it exploded. Four people were injured, including a policeman and our security guards," she added.
"I saw clearly that a policeman's hand was cut off and our security guards had shrapnel wounds," she said.
Religion has been under the spotlight in recent weeks in Indonesia, the world's most populous Muslim country, after some provincial administrations issued a local decree which banned members of the Islamic Ahmadiyah sect from showing signs identifying their mosques and schools.
End item/ 149