Hassab said that one of those sentenced to death is aged 16 - a punishment that would be illegal in Sudan - but the judge "thought the suspect lied about his age".
"The judge found them guilty of the charges against them and gave them one week to appeal the decision," he said.
"This is not fair because we are already at the end of the [working] week. Then we have two days weekend. So we just have five days to appeal against the verdict," Hassab said.
In the other court, in Khartoum North, Osama Osman, the judge, found 10 men guilty under Sudanese criminal law and counter-terrorism legislation and referred the cases of three suspected fighters to juvenille courts.
"I sentence you to death by hanging and you have the right to appeal this decision within 15 days," he addressed those standing in the dock.
The condemned, aged around 18 to 25 years old, broke out into loud cries of "God is great" and "thanks be to God", waving their arms in the air.
Saleh Balila, one of the defence lawyers, vowed to appeal against the sentences as quickly as possible.
Sudan has put on trial dozens of suspects over the May attack, the first time that decades of regional conflict between government forces and rebels reached the capital.
Defence lawyers argue that the special courts are unconstitutional and do not guarantee their clients' legal rights.