Egypt’s military council has said the decision to dissolve the country’s parliament must be upheld, after new President Mohammed Mursi ordered the assembly to reopen.
The military said it was confident “all state institutions” would respect the constitution and the law.
The intervention is seen by some as a challenge and warning to the president, who was sworn in only a week ago.
The BBC’s Jon Leyne, in Cairo, says the political truce in Egypt appears over.
The military council’s words will infuriate members of the Muslim Brotherhood, who believe it was the military themselves who failed to respect the law when they gave new powers to themselves after dissolving parliament last month, our correspondent says.
Crowds are already beginning in gather in Tahrir Square ahead of a parliamentary meeting called for 10:00 (08:00 GMT) on Tuesday, in defiance of the military’s decision.