Egypt's Supreme State Security Prosecution ordered on Sunday the detention of Islamists Mohamed El-Zawahiri and Mostafa Hamza for 15 days on charges of joining an "organised terrorist group.”

El-Zawahiri, brother of Al-Qaeda leader Ayman El-Zawahiri, was arrested in Cairo on Saturday. He is known to be a leading member of Egyptian hardliners the Salafist Jihadist group. Hamza, a member of Al-Gamaa Al-Islamiya, another ultraconservative Islamist grouping, was also arrested on Saturday.

The prosecution accused the two men of joining an "illegal and terrorist" group to "assault citizens' personal freedoms, harm social peace and national unity and prevent state institutions from performing their duties." However, it is unclear if the charges are related to the groups the detainees are known to be members of.

Both men, who were questioned in detention, denied the accusations against them.

El-Zawahiri, who was sentenced to death in 1999 by an Egyptian military court on terrorism charges, was released from prison in Egypt in 2011 following the revolution. In 2012 he was acquitted of the charges in a retrial.

Similalry, Hamza was also sentenced to death in absentia previously on charges of plotting to assassinate ousted president Hosni Mubarak during his visit to Ethiopia in 1995. However, he was acquitted in April following a retrial.