Egypt's military rulers invited parliament's two houses to convene on Saturday to elect an assembly tasked with writing the country's first constitution since the overthrow of Hosni Mubarak.

A tug-of-war has already begun over the shape of the future document that will define the balance of power between the army-backed executive and parliament, which wants to curb broad presidential powers.

Under an interim constitution, parliament is responsible for picking the 100-strong assembly that will write the new constitution to replace the one that helped keep Mubarak in power for three decades.

Political parties, led by the Muslim Brotherhood's Freedom and Justice Party which emerged as a key player in the new parliament, have already been in talks over the make-up of the constituent assembly.

The parliamentary vote for the upper house finished last week, concluding the first elections since the popular uprising toppled Mubarak a year ago.

The earlier lower house election, which started in November, saw an unprecedented turnout and was hailed as Egypt's most democratic since military officers overthrew the king in 1952.

But an Egyptian court ruled last week that the voting system that elected the new parliament was unconstitutional, creating a fresh source of uncertainty which could hold up the functioning of the new legislature.

It is not clear whether the 100 members of the assembly will be selected just from parliament.

Under the expected arrangement, the assembly will include legal experts as well as members of parliament.

(Reporting by Dina Zayed; Editing by Alison Williams)