Egypt's parliament has for the second time approved the convening of a constitutional assembly after the first attempt was criticised for including too many Islamists.

But the list of 100 constitution drafters immediately triggered similar objections from liberals and Christians, raising the prospect of fresh legal challenges to the new assembly in the courts - the latest hurdle in Egypt's bumpy transition to democracy.

The delays mean the new president will not know the extent of his powers when he is elected in a run-off vote this weekend.

Islamists hold about two-thirds of the seats in parliament, leading to fears among liberals that they will again be sidelined in the new Egypt, despite their contribution to the overthrowing of Hosni Mubarak's autocracy last year.

The presidential run-off election adds to those fears, pitting Mohamed Mursi, a member of the Muslim Brotherhood, against Ahmed Shafik, a former air force general who was Mubarak's last prime minister.

The main parties in parliament said last week that they had reached agreement on the shape of the constitutional assembly, and both houses approved the list late on Tuesday.

"This assembly saw many twists that hindered it but in the end it represents all groups," parliamentary speaker Saad al-Katatni told a joint session of both houses.

Katatni, who resigned from the Muslim Brotherhood's party to take the speaker's post, said the list included 33 people from political parties, including members of parliament, as well as constitutional experts, judicial figures, Christian and Muslim clerics, union members and representatives of the army, police, government and Egypt's youth.

However, some liberal and independent members of parliament walked out in protest on Tuesday, saying the list under-represented women, intellectuals, and Christians, who make up 10% of Egypt's 82million people.

Amen Eskander, a member of parliament for the Al-Karama Party, said there would be too many Islamists in the assembly, "just like in the previous one".

Egypt's Coptic Orthodox, Anglican and Catholic churches withdrew their representatives from the first assembly but have indicated that they will remain in the latest one. Together these churches secured seven seats.

Egypt's prestigious al-Azhar seat of Sunni learning has five seats. It also withdrew from the first body in solidarity with liberals, churches and others.

The ruling generals have pledged to hand power to a new president by July 1 as the climax of almost a year-and-a-half of messy and often bloody transition to civilian rule, but the failure to establish a clear path towards a new constitution suggests more turbulence ahead.

Among the issues are the extent of presidential and parliamentary powers and the degree to which Islamic law will be applied.