* Former president toppled by army in July
* Mursi supporters tried to march to protest camp
* Say will demonstrate until army-backed government falls (Updates with number of protesters arrested in paragraph 4)
By Michael Georgy
CAIRO, Oct 20 (Reuters) - Egyptian security forces fired bird shot and teargas to prevent supporters of deposed Islamist President Mohamed Mursi from marching on Sunday to the site of a protest camp that was destroyed two months ago, a Reuters witness said.
The crowd of about 100 people were students from Al-Azhar University, the historic seat of Sunni Muslim learning. They threw rocks at riot police stationed outside the gates of the university, and police threw the stones back.
The university is in the same Cairo suburb as the Rabaa al-Adawiya mosque, scene of one of two pro-Mursi protest camps crushed by security forces on Aug. 14. Hundreds of protesters were killed.
"Rabaa Square is completely off-limits," a security source said. "Protesters are not allowed to move inside it." A separate security source said 11 students had been arrested.
Authorities have cracked down hard on Mursi's Muslim Brotherhood. The group was outlawed by a court order after the army overthrew Mursi and installed an interim government in July following massive street protests a year after his election.
Brotherhood leaders, including Mursi, have been arrested on charges of inciting or taking part in violence.
Brotherhood supporters say they will keep protesting until the army-backed government falls. But demonstrations are far smaller than the ones that immediately followed Mursi's downfall.
The Interior Ministry said in a statement that security forces took action after 3,000 students blocked roads around the Al-Azhar campus.
A few police trucks kept students from moving beyond the front entrance of the facility.
"We want the return of legitimate rule to Egypt, we want the return of President Mursi" said Mohamed Magdi, a commerce student. "We are unarmed students. We just approached them and said 'you are our police' and then they attacked us."
The students had been protesting for the second day on campus in support of Mursi. Graffiti scrawled on university buildings condemned General Abdel Fattah al-Sisi, the army chief who toppled Mursi, as a murderer and traitor.
"We will take measures to continue studies even with the continuation of the demonstrations," said Ibrahim el-Hadud, a university official.
The army rejects allegations from the Brotherhood that it deposed Mursi in a coup and says it was responding to the will of the people.
The government refers to the Brotherhood as "terrorists" and does not distinguish between the movement and al Qaeda-affiliated groups in the lawless Sinai Peninsula who carry out almost daily attacks on security forces.
The Brotherhood describes itself as a peaceful movement. (Additional reporting by Yasmine Saleh; Writing by Maggie Fick; Editing by Yara Bayoumy and Alison Williams)