Egyptian lawmakers held a heated session on Saturday as they debated how parliament would pick members of an assembly tasked with writing a new constitution, a process crucial to democracy there after the ousting of Hosni Mubarak.

The document will define the balance of power between the army-backed executive and parliament, which wants to curb broad presidential powers and may become the focus of confrontations over the role of Islam in Egyptian law and society.

The constitution will also outline the future political role of the military, which has been in power since Mubarak was toppled in a popular uprising in February 2011.

Under an interim constitution, parliament is responsible for picking the 100-strong assembly that will write the new constitution, replacing the one that helped keep Mubarak in power for three decades and was a cornerstone of his rule.

The most important step to building democratic institutions is what we are about to do here today, Saad al-Katatni, the speaker of the lower house of parliament, said at the start of a joint session between the two houses.

The path of our revolution was not paved with flowers but with sacrifices, he said after asking lawmakers to read a verse of the Koran in honour of the victims of the uprising.

The speaker suspended the session on Saturday, agreeing that parliament would form a smaller committee to receive proposals on how the constituent assembly could be formed. The two houses will then meet again on March 17 to discuss the proposals and finalise the selection criteria. They will vote on the assembly a week later on March 24.

MUSLIM BROTHERHOOD IN DRIVING SEAT

As members of parliament in the Arab world's most populous state outlined their visions for the make-up of the body, early signs of disagreement were evident.

The Muslim Brotherhood's Freedom and Justice Party (FJP), which controls most seats in both houses of parliament, wants 40 members of the constituent assembly to be from parliament and the remaining 60 to include legal and constitutional experts and members of unions.

That vision is opposed by liberal groups in parliament, who stress that women, young people and Christians should be fairly represented and who seek to draw from outside parliament to strengthen their influence.

Both the liberals and the Brotherhood's Salafi rivals, which call for a strict application of Islamic law, worry the FJP may be able to pack assembly members coming from parliament with its own lawmakers.

To counter this, the Salafi al-Nour Party wants more MPs in the assembly than advocated by the FJP.

Those in a better position to be in the assembly should be the elected members of parliament. They should have a priority over others, said Nour's Moustafa Khalifa.

The FJP says it wants an assembly that can represent all parties, and a constitution that can safeguard Egypt's freedoms.

We want an assembly that represents all of Egypt's people and we will cooperate with everyone from inside parliament and outside to do so, said Hussein Ibrahim, an FJP lawmaker.

DIFFICULTIES AHEAD

One FJP member of parliament sought to defuse worries about the make-up of the assembly and the dominance of his own party. Mohamed el-Beltagi said he foresaw a broad consensus on the articles of the new text.

It is impossible for any party, any authority or any sect ... to have some form of guardianship or to usurp others in this nation at this phase. Nor can anyone seek to marginalise or sideline another, he said.

Once the assembly is formed, it has six months to draft the constitution, which will then be put to a referendum.

But Saturday's fierce debate in deciding the criteria by which assembly members will be chosen may be a foretaste of the difficulties in writing the document.

Political parties aim to play a significant role in the constitution-writing process and some commentators fear hard-line Islamists may seek to curb citizens' freedoms.

The assembly will have to try to establish what role to give the military, which for decades has had no civilian oversight. Some Islamists are expected to defend the status of the military in what political watchers suspect is a tacit deal between the two.

The process of granting Egypt a new constitution will not be an easy one. Egyptians need real debate about their constitution, liberal MP Amr Hamzawy told Reuters this week.

Some lawmakers kicking off the process on Saturday stressed that expertise, rather than party affiliation, should be the crucial factor when selecting members of the assembly.

Picking the constituent assembly should be governed by expertise and not numbers, it should be about caring for substance and not appearances, Essam Sultan, a lawmaker with the moderate Islamist Wasat party, told parliament.

(Additional reporting by Omar Fahmy; Writing by Dina Zayed; Editing by Alessandra Rizzo and Ben Harding)