The main protest rally was due to take place after prayers at the Saida Aisha mosque, in a working class district close to the ancient citadel.

Some 2,000 anti-riot police were deployed in the square nearby and a police officer at the scene said "the mosque is closed, there is nobody inside and by staying here you could get arrested."

Water cannons and prison vans were visible in the square near the mosque, in an apparent show of force to deter any gathering.

Besides baton-wielding anti-riot police, the square was empty, in sharp contrast to the crowded streets nearby.

Three opposition parties -- the liberal Wafd, the Arab Nasserite Party and the leftist Tagammo -- said Wednesday they had obtained permission to demonstrate, claims denied by the interior ministry.

"If a demonstration is held, the interior ministry will arrest its organisers," the government said in a statement published in Friday's newspapers.

The Muslim Brotherhood, the largest opposition group, said Thursday it would only take part in authorised demonstrations.

Police also barred all access to the central Tahrir square, a traditional magnet for demonstrators, and manned road blocks set up on the first day of the war, on March 20, around the US and British embassies, not far from the square.

Hundreds of policemen were also deployed around Al-Azhar mosque, one of the most cherished in the Islamic world, keeping an eye on thousands of worshippers as they left following prayer.

Last Friday, the interior ministry for the first time authorised an anti-war rally in the streets of Cairo, after prayers at Al-Azhar.

Emergency laws, which have been in force almost continuously since 1967, ban public protests. However, they are tolerated on university campuses and in mosque compounds.

Around 100 demonstrators and policemen were slightly injured in clashes on Tahrir square on the first day of the war. In the first three days following its outbreak around 800 demonstrators were arrested, according to the Egyptian Bar Association.

SPACE.WIRE