According to Norfolk County Police Department, the unplanned detonation happened about 17:00 GMT when army specialists were attempting to disarm it.
According to testimonies on social media, Internet users said they heard a loud bang and felt buildings shake 15 miles (24km) away.
The device -about 1m (3.2ft) long and weighing about 250kg (39st 5lbs) – was discovered on Southtown Road by a contractor working on the third crossing over the River Yare.
Specialists had built a protective sand barrier around the bomb prior to attempts to detonate it, as part of safety measures.
The cordons, at 200m (656ft) and 400m (1,312ft) from the bomb, were lifted on Friday evening.