Egypt's first Islamist president, Mohamed Mursi (R) meets with Field Marshal Mohamed Tantawi, head of Egypt's ruling Supreme Council of the Armed Forces (SCAF)
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, reports the BBC.
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.
MPs are due to gather for a formal session on Tuesday morning. They were allowed back in the building on Monday for the first time since parliament was dissolved and military guards placed outside.
That provided a glimmer of hope that a compromise might be possible. But with a big demonstration planned for Tahrir Square, the political temperature is rising by the hour.
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.
Earlier on Monday, the Supreme Constitutional Court rejected Mr Mursi's order to reconvene the Muslim Brotherhood dominated-parliament and said its ruling was binding.
But MPs were allowed back into the parliament building, for the first time since the assembly was dissolved.
As the court had not itself ordered the dissolution of parliament, Mursi was not directly challenging a court order, our correspondent says.
But while MPs could meet, any laws they passed were most likely to be ruled invalid, he adds.
Mursi won the country's first free presidential election last month, and army chiefs formally handed over power on 30 June.
The Supreme Council of the Armed Forces (SCAF) dissolved parliament in June, after the constitutional court ruled that part of the election for parliament was unconstitutional.
Before 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.
In his decree on Monday, Mursi said new parliamentary elections would be held 60 days after the new constitution had been agreed by referendum.
Despite the apparent tensions, the president and Field Marshal Hussein Tantawi, who heads SCAF, appeared together at a military cadet graduation ceremony on Monday.