They were demonstrating against the sacking of five of their colleagues who last week took over a sales garage in a protest demanding higher wages.
The workers attempted to force their way into the building but were prevented by steel doors.
Some workers rang the doorbells to the senior management offices but received no response.
The demonstrators carried red flags and banners and chanted slogans as they blocked the building's entrance.
None of company management came out to talk to the workers.
Martin Villams, who is a member of the Fiat union in Belgium, said it was "unacceptable" that the workers who staged last week's demonstration in the garage had been dismissed.
"If you can be dismissed for serious fault just because you take part in social action then you don't any more have any right for collective action and collective bargaining," he said.
Villams said Fiat must abide by the "rules of the social dialogue" and hold discussions with its workers about their demands for higher wages.
Fiat's CEO Sergio Marchionne said on Friday that the Italian automaker had not given up on reaching a deal for Opel, even though a rival takeover plan by bidders Magna International Inc. and Russia's Sberbank was moving ahead.