CAIRO, Egypt — The formerly banned Muslim Brotherhood, Egypt's best-organised movement, announced on Saturday that it has formed a non-theocratic party that will contest up to half of parliament's seats in September elections.

Mohammed Hussein, the group's secretary general, told a news conference the movement's council had decided to form the Freedom and Justice Party.

"We have adopted the measures taken by the guidance council regarding the Freedom and Justice Party and adopted its programme," he said.

He added that the party will contest 45-50 percent of parliament's 508 elected seats in the September polls, the first since a revolt ousted president Hosni Mubarak in February.

He did not say why the group had settled on that number.

The party, headed by Brotherhood politburo member Mohammed al-Mursi, will be "independent from the Brotherhood but will coordinate with it," he said

Mursi, who had run the Brotherhood's previous parliamentary campaigns, said the party was not "theocratic."

"It is not an Islamist party in the old understanding; it is not theocratic. It is a civil party."

Egypt's constitution bans parties based on religion, class or regionalism.

The Brotherhood has sought to allay fears that an Islamist parliamentary majority might emerge from the polls and said it would be willing to cooperate with secular groups in the September election.

It has also pledged not to field a candidate in a presidential election, to be held in November.

A tentative party programme leaked to the press in 2008 said that only an Egyptian Muslim male could be president of the country, causing a fire storm of criticism at the time.

Hussein said after the press conference that the Brotherhood remained of the view that the presidency could "only be undertaken by a Muslim male."

But Saed al-Katatni, Freedom and Justice's secretary general, said the party "would not object to any Egyptian" and added there was a "multiplicity of views" in the Islamist movement.

Brotherhood officials said the party's programme will be released at a later date.

In the past, the Brotherhood ran candidates as independents to circumvent a ban on the group in place since 1954.

It won a fifth of seats in a 2005, but fared badly in a November 2010 election that was widely seen as rigged to favour the former ruling National Democratic Party.

The group, which recently built a seven-story headquarters decorated with its symbol of crossed swords and a Koran, appears to have gained acceptance from the country's ruling military council after years of arrests by Mubarak's security forces.

But other groups who took part in the 18-day revolt that ended Mubarak's regime have expressed fears of the Brotherhood sweeping to power in an election that secular groups would be too disorganised to contest.

Its announcement that it would contest up to half of the seats in parliament will dismay other political groups, said Diaa Rashwan, an expert on Islamist movements with the Ahram Centre for Political and Strategic Studies.

"This announcement will be depressing for political forces in Egypt," he said, adding that other Islamist factions who said they would participate might field candidates for the 50 percent of seats.

But Rashwan said he did not expect the Brotherhood to gain more than a fifth of seats and that the group's clout was exaggerated under Mubarak's regime.

"The revolution was not Islamist. After the revolution, we have to revise the idea that the Brotherhood was the strongest force in the country," he said.