At least 42 people were killed on Monday during an attack on supporters of ousted Egyptian president Mohamed Morsi outside an elite army base in Cairo, a senior medical official said.

"The death toll is 42 dead and 322 wounded," Ahmed al-Ansari, the deputy head of emergency services, told AFP.

The Muslim Brotherhood, which has led pro-Morsi demonstrations, said 35 of its supporters were killed when police and troops fired at them while they were praying at dawn.

Witnesses, including Brotherhood supporters at the scene, said the army fired only tear gas and warning shots and that "thugs" in civilian clothes had carried out the deadly shooting.

Armed supporters of Morsi meanwhile seized two soldiers in Cairo, an army official said.

Both of the Egyptian soldiers, who were identified as Samir Abdallah Ali and Azzam Hazem Ali, were put in a vehicle and forced to make pro-Morsi and anti-army statements on a loudspeaker, the official said, cited by state news agency MENA.