Smoke could be seen rising from close to the NOC offices and security officials said they were attempting to deal with militants believed to have attacked the building.
Security forces took up positions around the offices in central Tripoli and surrounding roads were cordoned off.
A Reuters witness saw ambulances leaving the site. A member of staff from a hotel next to the NOC offices said he had heard about five blasts.
Tripoli was shaken by clashes between rival armed groups at the beginning of this month but the capital has also seen occasional militant attacks.
In May, Islamic State claimed a deadly attack on the national election commission offices in Tripoli.