(Reuters) - An Egyptian court on Tuesday halted a move to create a new constitutional assembly pending a ruling on its legality, challenging the legitimacy of a body spurned by liberal and Christian groups because of its domination by Islamists.

The injunction slapped on the decision by parliament could delay the introduction of a constitution needed urgently to clarify the powers of Egypt´s new head of state, due to take over from ruling generals by mid-year.

Lawyers for the state say the court has no say over the move to form the constituent assembly, which has pressed ahead with its work even though dozens of non-Islamist representatives have quit, complaining that their voices are being drowned out.

The latest dent to the body´s authority drew a measured response from the Muslim Brotherhood, the Islamist movement which holds most seats in parliament and chairs the assembly.

"I respect all rulings and call on all national forces to sit together to reach better solutions to overcome this crisis," Brotherhood presidential candidate Khairat al-Shater said in a statement on the movement´s website.

The court´s judge, Ali Fekri, rejected legal arguments saying it was not qualified to rule on the issue and said it had decided to halt the decision that formed the assembly. He passed on the case´s documents to a judicial panel for a review.

The case, brought by lawyers and activists, follows several lawsuits demanding the dissolution of the assembly on the grounds that it fails to represent Egypt´s diversity.

"We expect parliament to appeal the verdict before the supreme administrative court but until the appeal gets ruled on, the current constitutional committee has no existence," said the main lawyer in the team who filed the case, Essam al-Islamboly.

BOYCOTT

Selected by parliament, the 100-member constitutional assembly is composed mostly of Islamists, who won the majority of seats in Egypt´s first free parliamentary vote in decades.

Only a handful of seats were reserved for youth groups, women and for Christian Copts, who said they plan to boycott the assembly, following the example of liberal groups and the country´s highest Sunni Islamic institution al-Azhar, who all withdrew from the assembly.

A representative of Egypt´s Supreme Constitutional Court also withdrew in protest.

"This ruling will allow the Egyptian people to protect the January 25 revolution from the hegemony of the Islamist current and from the Muslim Brotherhood that seeks to polarize and bar civilian politicians who participated in the revolution," rights activist Youssef Abdel Khalek.

A top judge, Zaghloul al-Balshy, vice chairman of the Court of the Cassation, told Reuters that Tuesday´s court decision meant "the make- up of the assembly has to be reconsidered and the new representation has to represent the entire society."

The current constitution was suspended by the army in February last year, shortly after it took power from Egypt´s long-serving autocratic president, Hosni Mubarak, who was forced out by a popular uprising.

The new constitution will define the balance of power between parliament and the president, the influence of Islamic sharia law over statute and society and the extent of the army´s influence over the government and freedom from supervision.

An army representative attending the assembly´s second meeting last week urged political powers to put their differences aside and said the constitution writing process would not delay handing power to an elected president.

(Additional reporting by Sherine El Madany and Tamim Elyan; Writing by Tom Pfeiffer; Editing by Myra MacDonald) (© Thomson Reuters 2012. 04/10/12)