An unexploded WWII bomb discovered near tracks leading to France's busiest train station has severely disrupted rail service as police work to disable the device.

The unearthing of a 500-kilogramme World War II bomb near tracks outside Paris on Friday halted traffic and stranded thousands of passengers in one of the biggest rail disruptions in the French capital for years.

All traffic to the train station, which hosts Eurostar trains as well as high-speed and local services, was closed as Paris police worked to disable the device.