The lawyers were jailed in Marsa Matruh for assaulting three court workers there on January 5.
The union also told its members not to answer the questions of prosecutors.
A court in the Mediterranean city convicted the six lawyers of assault and sentenced them to two years in jail on Wednesday.
The union says that judicial authorities there did not observe due process and is demanding their acquittal.
It said: “There was inexplicable insistence and a clear display of intransigence when pre-trial detention of our colleagues was ordered, although the requirements for such a decision did not exist in their case.
“They were also hurriedly referred to trial without appropriate questioning.”