"God is Great! Jem is strong! Revolution, revolution until victory!" the defendants shouted after hearing the verdict.
Three others were acquitted and will be freed, the court said.
More than 220 people were killed when Jem fighters reached Khartoum's twin city of Omdurman, more than 1,000km from the western Darfur region where it has been fighting government forces and allied Arab militias.
Sudan set up special courts in Khartoum, Khartoum North and Omdurman to try dozens of suspects rounded up in a security clampdown following the attack.
Defence lawyers have argued that the courts are unconstitutional and do not guarantee their clients' rights.
At least 50 other Jem fighters are already awaiting execution over the attack.
"This judgment is illegal, illegitimate and violates international law," Ahmed Hussein, a Jem spokesman, told the AFP news agency after the verdict.
"This is yet another evidence that the judicial system of the regime is not independent."
Hussein said that the trial was also a "clear violation" of an agreement reached in Qatar which aimed to end the fighting in Darfur, which the UN says has killed up to 300,000 people and forced more than 2.2 million to flee since 2003.
However, Jem pulled out of those peace negotiations with the government last month after al-Bashir expelled 13 international aid agencies.
The Sudanese president made the move after the International Criminal Court ordered his arrest on charges of war crimes and crimes against humanity in Darfur.
The Jem fighters are predominantly from the Zaghawa tribe, one of three ethnic groups - along with the Masalit and Fur - that al-Bashir is accused of trying to wipe out.
The war in Darfur began in February 2003 when ethnic minority fighters took up arms against the Arab-dominated Khartoum government and state-backed militias, fighting for resources and power in one of the most deprived regions on earth.