Norwegian news site NRK said police spotted a suspicious man in the area, questioned him him and then found the device.
Norwegian police have sealed off a large area of the Gronland neighborhood in downtown Oslo after a suspected explosive device was discovered late Saturday.
When asked about the controlled explosion, a police spokesman said: "The noise from the blast was louder than our explosives themselves would cause".
The Groenland area is a multi-ethnic neighbourhood which is home to several popular bars and restaurants.
According to witnesses, police evacuated people from surrounding restaurants and bars as the investigation proceeded. Police announced that they had a suspect in custody in relation to the found device.
The incident came just one day after a truck ploughed into pedestrians on a busy shopping street before crashing into a department store in central Stockholm, Sweden, killing at least four and wounding many others. "The police were screaming at us to get back inside and stay where we were", she added by telephone. It was unclear whether he was also a Swedish citizen or resident or even how long he'd been in the country.