Local residents are being advised to remain indoors with their doors and windows closed while the incident is ongoing.
A road closure is also in place at the nearby Michaelson Bridge.
Police said the Devonshire Dock Hall, the site’s main building facility, had been evacuated and that everyone inside had been accounted for.
Covering around six acres, the hall is the second-largest indoor shipbuilding complex of its kind in Europe, according to BAE Systems.
The craft to have been built in the hall include the four Vanguard Class submarines that make up the UK’s Trident nuclear programme.
