Smoke fills the area during clashes between supporters of Egypt's deposed president Mohamed Mursi and riot police in Cairo January 17, 2014. [Photo/Agencies]

State authority collapsed in parts of the Sinai peninsula after the downfall of veteran president Hosni Mubarak in 2011, allowing hardline Islamist groups to expand into the vacuum.

Attacks on policemen and soldiers in the peninsula, which shares borders with the Palestinian Gaza Strip and Israel, intensified after the army ousted Egypt's Islamist president Mohamed Mursi last July amid mass protests against his rule.

Egyptian security forces have arrested thousands and killed hundreds of Brotherhood supporters since the overthrow of Mursi, Egypt's first democratically elected leader, and last month they declared the group a "terrorist organisation".

The Brotherhood, which says it is committed to peaceful activism, had unsuccessfully urged a boycott of the referendum on a new constitution.

State media, citing initial estimates, said around 95 percent of voters supported the new constitution, which would replace one approved under Mursi and would strengthen the state bodies that defied him: the army, the police and the judiciary.

In another sign of pressure on the Brotherhood, members of the engineers' union forced their head, identified as a Brotherhood supporter by state news portal Al-Ahram, to resign.

Unions have traditionally been seen as a gauge of Brotherhood support, in large part because the group was banned from politics during the Mubarak era. It lost its grip on another powerful professional union, representing doctors, in a vote last month.