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.
'Binding and final'
The statement from the Supreme Council of the Armed Forces (Scaf) will infuriate the Muslim Brotherhood, our correspondent says.
Members of the Brotherhood believe it was the military that failed to respect the law by giving itself new powers after dissolving parliament last month, he says.
Earlier on Monday, the Supreme Constitutional Court rejected the decree issued by Mr Mursi the day before to reconvene the Muslim Brotherhood-dominated parliament.
The court said its 14 June ruling - that part of last year's parliamentary election was unconstitutional - was binding and final.
But army units posted outside the parliament building had already left and some MPs had gone in for the first time since the assembly was dissolved.
As the court had not itself ordered the dissolution of parliament, Mr Mursi was not directly challenging a court order, our correspondent says.
No mention was made of the court's ruling in the decree. And presidential spokesman Yasir Ali argued that Mr Mursi had been quite legitimate in suspending the dissolution until new parliamentary elections took place within 60 days of a new constitution being ratified.
Despite the apparent tensions, the president and Field Marshal Hussein Tantawi, who heads Scaf, appeared together at a military cadet graduation ceremony on Monday.
'Waste of authority'
The president's order has not, however, been welcomed by political rivals.
Former presidential candidate Abdul Moneim Aboul Fotouh was said to have denounced his move as unconstitutional while another, Hamdin Sabbahi, was quoted as saying it was a "waste of legal authority".
Liberal MP Mohammed Abu Hamed urged Scaf to challenge what he called "this constitutional coup".
The constitutional court is due to hear a number of appeals against the decree on Tuesday, reports say.
Mr Mursi won the country's first free presidential election last month, and army chiefs formally handed over power on 30 June.
Before Mr Mursi's inauguration, the military granted itself sweeping powers.
The commanders' constitutional declaration stripped the president of any authority over the military, gave military chiefs legislative powers, and the power to veto the new constitution, which has yet to be drafted.