The demonstration at the plant erupted after the company board announced a restructuring plan entailing the loss of 284 jobs by the end of 2006, Belga news agency reported.
The factory at Fleurus, near the southern industrial town of Charleroi, currently employs 844 people making car windscreens and windows.
AGC Automobile Europe, formerly known as Splintex, said in a statement that the restructuring plan has the "overarching objective of ensuring its viability in an extremely competitive market".
But unions at the Fleurus plant complained that the board had failed to consult them and so called on their members to down tools, leading to the directors being confined to their offices by the protest.
AGC Automobile is one of 13 European affiliates of Asahi Glass, the world's biggest glass maker, and has eight factories in Europe with Fleurus the largest.