Activists who were at the forefront of the uprising that toppled veteran Egyptian leader Hosni Mubarak in February are calling for new protests on Friday to press the country's military rulers to keep their promises of reform.
The Coalition of Revolutionary Youth urged Egyptians to take to the streets to demand a clear timetable for the restoration of civilian rule and for a halt to the routine use of military courts to try cases involving civilians.
The largest demonstrations are expected in Cairo's Tahrir Square, the epicentre of the mass rallies that ended Mubarak's three decades in power.
The Supreme Council of the Armed Forces issued a statement late on Wednesday warning it would "deal firmly with any attempt to create disturbances."
It cautioned in particular against "any attack on military units, personnel or vital facilities."
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The council's chairman, Field Marshal Hussein Tantawi, tried to reassure activists about the military's intentions in a speech on Thursday.
He pledged to "organise parliamentary and presidential elections in a completely transparent and impartial manner."
"In the current circumstances it is vital that everyone defend the stability of our country," he added.
The Coalition of Revolutionary Youth urged its supporters to "avoid any actions that could spark chaos" and to be wary of "infiltration by elements seeking to stir trouble."
The Muslim Brotherhood, by far the best organised opposition group under Mubarak's regime, said it would not join the protests but added that it would not cede the initiative to secularist groups.
Secular politicians have voiced concern that the military's current timetable for parliamentary elections in October and November will play into the hands of the Brotherhood by denying new political movements the time to organise on the ground.