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 closed parliament last month after a court ruling.
Its latest intervention is seen by some as a challenge and warning to the president, sworn in barely a week ago.
The military council said it was confident "all state institutions" would respect the law and constitution.
The BBC's Jon Leyne, in Cairo, says the political truce in Egypt appears over.
Crowds were gathering in Tahrir Square ahead of a parliamentary meeting called for 10:00 (08:00 GMT) on Tuesday, in defiance of the military's decision.
BBC News - Egypt military 'warns' President Mursi over parliament decision