RIYADH - A new Islamist group that calls the army's ousting of Egyptian President Mohamed Mursi a declaration of war on the faith, and which threatens to impose Islamic law by force, has announced its formation.

The group, Ansar al-Sharia, said on Friday in a posting on an online forum for militants operating in Egypt's Sinai region that it would accumulate weapons and train its members in their use.

It blamed the revolt against Mursi's rule on "secularists", supporters of former president Hosni Mubarak and on Egyptian Coptic Christians.

The group denounced democracy as "an abomination conceived by the Crusaders [Christians and the West]" and said it would enforce Islamic law, sharia.

The army's move, which was backed by the reformists who held mass rallies across Egypt, has raised fears that Islamists will desert relatively moderate groups such as Mursi's Muslim Brotherhood and move to extremist movements.

Mursi's ousting has already triggered violence. At least 24 people were killed as Islamists took to the streets in Cairo and other cities on Friday to vent their fury at what they say was a military coup.

Mursi was elected president last year after a popular revolution swept away veteran leader Mubarak.

The army has appointed an interim leader and announced a transition plan that lacks a date for elections. It has arrested senior Muslim Brotherhood members and closed Islamist television stations.