(Adds Turabi party HQ surrounded, govt reax)

By Opheera McDoom

KHARTOUM, May 12 (Reuters) - Darfur rebel leader Khalil Ibrahim said on Monday he would launch more attacks on Sudan's capital Khartoum until the government fell.

"This is just the start of a process and the end is the termination of this regime," Ibrahim, whose Justice and Equality Movement (JEM) attacked Khartoum at the weekend, said in a satellite phone call. "Don't expect just one more attack."

Ibrahim said he was speaking from Omdurman, the western Khartoum suburb where the attack occurred -- just across the Nile river from the heart of the capital.

Government officials have said the last rebels fled the area on Sunday evening, and no independent verification of Ibrahim's statement about his whereabouts was immediately available.

The weekend attack, which Sudan says was supported by neighbouring Chad, was the first time fighting had reached the capital in decades of conflict between the traditionally Arab-dominated central government and rebels from far-flung regions in the oil-producing nation -- Africa's biggest country.

President Omar Hassan al-Bashir's government arrested Islamist opposition leader Hassan al-Turabi and at least four other top members of his party on Monday, aides said.

JEM also has an Islamist agenda and some of its leaders were allies of Turabi in the past, but he denies backing the rebels.

Turabi's son said security forces arrested his father at his home about an hour after returning from a conference of his Popular Congress Party (PCP) in nearby Sennar state.

"They want to blame the party for what has happened," said Siddig al-Turabi. About 65 people were believed to have been killed in the Khartoum attack.

Seven heavily armed vehicles surrounded the PCP head quarters on Monday, a witness said. Party officials said more members were being arrested.

POWER STRUGGLE

Turabi was Bashir's ideologue until they split in a bitter power struggle in 1999-2000. Since then he has been in and out of jail but was released along with all other political prisoners after a 2005 north-south peace deal.

Mutrif Siddig, under-secretary at the ministry of foreign affairs, told Reuters he doubted Ibrahim was in Omdurman and that the government was ready for any further attack.

"We have some lessons learned and we will be better prepared if he dares to do so," he said.

The rebels made a lightning advance across 600 km (400 miles) of desert and scrub from the western Darfur region to attack Khartoum on Saturday in what one of their leaders called a bid for power.

On Sunday, Sudan cut diplomatic relations with Chad, saying the rebels were supported by Chadian President Idriss Deby.

Chad has denied involvement and expressed surprise at Sudan's "hasty decision", but analysts say it may have backed the JEM rebels to retaliate for an attack on the Chadian capital three months ago.

Deby and Bashir signed a non-aggression pact in March. Each has accused the other of breaking the deal.

Rebels in Sudan have complained for decades of neglect by the central government.

A peace deal between north and south ended one civil war in 2005 and boosted Sudan's economy by increasing oil production in the south, but that agreement did not cover the conflict that erupted in Darfur five years ago.

International experts estimate some 200,000 people have been killed and that 2.5 million have been made homeless in Darfur since mostly non-Arab rebels took up arms.

Government officials said the attack on Khartoum ruled JEM out of any peace process.

A curfew remained in force on the outskirts of Omdurman. Military checkpoints were at every major junction.

Heavy tanks lined Omdurman's streets and dozens of vehicles carrying armed men raced along. Security forces were arresting mostly young men who looked to be from Darfur.

But the streets were again full of people. (Editing by Dominic Evans)

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