CAIRO, Sept 13 (Reuters) - Al Qaeda leader Ayman al-Zawahri has described the army-installed government's crackdown against the Muslim Brotherhood in Egypt as a "brutal crime" and urged Egyptians to resist what he called a campaign against Islam in their country.

Security forces have killed hundreds of Brotherhood supporters and arrested thousands, including most it leaders, since the army toppled Islamist President Mohamed Mursi on July 3 following mass protests against his rule.

Many were killed when the security forces, who regard the Brotherhood as terrorists, killed hundreds of its supporters on Aug. 14 in a raid on pro-Mursi protest camps in Cairo.

In an audio speech released a day after the 12th anniversary of the 9/11 strikes on the United States, former doctor Zawahri, condemned the break up of the camps and the arrest of Islamists.

"This is an episode of a long drama that awaits Egyptians if they don't unite to implement sharia Islamic law and free their country," said Zawahri, an Egyptian who was tortured by authorities during the rule of ousted strongman Hosni Mubarak.

Zawahri also said the United States was conspiring in Egypt and called on Egyptians to fight the "oppression".

Egypt, a U.S. ally and the most populous Arab state, faced an insurgency by supporters of Zawahri in the 1990s. Mubarak crushed those Islamists, who later renounced violence.

Egyptian state television on Thursday accused the Palestinian Hamas of training Egyptian Islamists in how to carry out bombings.

That signalled the authorities intend to pile even more pressure on the Brotherhood, a Hamas ally which emerged from the shadows to win every election since a popular revolt toppled Mubarak in 2011.

In neighbouring Gaza, the ruling Hamas Islamists denied the allegations.

Militant attacks have surged since Mursi's overthrow. Most of the violence is limited to Sinai, a relatively lawless region adjoining Israel and the Gaza Strip. Attacks on security forces there take place almost daily and about 50 personnel have been killed since Mursi was deposed.

Concerns are growing that the attacks will spread to Cairo, especially since a Sinai-based militant group claimed responsibility for a failed suicide bombing against the interior minister last week. (Reporting by Michael Georgy; Editing by Angus MacSwan)