Police had to be called and the protesters were removed with the help of bolt cutters.
Gratte Brothers said a female member of staff was pushed to the floor during the demonstration.
Gratte staff stopped the sparks in reception and prevented them from invading the rest of the office.
The demonstration was the latest round in a wave of protests against plans by leading M&E contractors, including Gratte Brothers, to quit the JIB pay and conditions agreement.
Rank-and-file electricians are now threatening to stage more sit-ins targeted at contractors and clients.
One eye-witness told the Enquirer: “There were about 50 lads who went into the office and chained the doors.
“It was all peaceful but the police were called and they were cut free with bolt cutters.
“The lads have decided to take more direct action because just standing outside sites only gets you so far.
“They are saying that clients of the big contractors could be next.”
A spokesperson for Gratte Brothers said: “This was not a peaceful protest. It was violent and abusive.
“Female staff members found this very intimidating.
“This was an act of trespass by a group of people who do not represent our workforce or the workforce of other contractors involved in the BESNA process.”