The explosive was discovered during construction work in Europa Allee, FAZ reported.
In the course of diffusing the bomb – weight of which hasn't yet been revealed – extensive evacuation measures are to take place within a radius of 700 meters until about 8 pm.
About 16,000 people are affected, according to Frankfurt police.
According to a spokesman for Deutsche Bahn, the trains will no longer stop at the Messe (or Frankfurt trade fair, the world's largest) starting from 7 p.m. It's not yet known if the line will have to be completely closed off.
The work to make the bomb safe is due to be completed by 1 a.m. Anybody who needs assistance due to not being able to access the area surrounding where the work is taking place can call 069/212111, reported FAZ.