By Marwa Awad

CAIRO (Reuters) - The Muslim Brotherhood's political party, Egypt's biggest, chose veteran conservative Saad al-Katatni as its new leader on Friday to replace Mohamed Mursi who went on to become his country's first elected president.

Katatni, 61, a microbiologist who joined the Islamist movement in 1979, is seen as more conservative than his main challenger for the post, Essam el-Erian, and less ready to compromise with liberals and leftists.

He was speaker of the first parliament after a popular uprising toppled autocratic president Hosni Mubarak, an assembly dominated by the Brotherhood's Freedom and Justice Party (FJP) and ultraconservative Salafis.

Liberals occasionally walked out of the assembly, protesting that their Islamist opponents were intent on railroading through their legislative agenda while ignoring the views of others.

The legislature was dissolved by court order in June after the rules of the parliamentary election were challenged. A court is to rule on October 23 on whether the assembly that is to draft Egypt's new constitution is legal.

Katatni took 67 percent of 866 votes by members of the FJP's general committee, or executive, on Friday.

Analysts expect no major policy change within Egypt's best-organized Islamist group, saying it was Katatni's calm, consensual stance within the movement that won him the leadership vote.

"I am indebted for this trust," Katatni said in a speech after the result was announced. "Egypt is waiting for us, the Freedom and Justice Party, to lead the political scene."

He said the role of the FJP, which has around 400,000 members, was to "implement righteous rule based on Islamic sharia laws," prompting loud applause from his audience.

(Additional reporting by Mohamed Abdellah and Omar Fahmy, writing by Yasmine Saleh, editing by Mark Heinrich)