By Opheera McDoom

KHARTOUM (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.

But there was no independent verification of Ibrahim's whereabouts but authorities said they were reimposing an indefinite curfew in Omdurman while security forces searched for rebels in the area.

The weekend attack 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.

Sudan said neighbouring Chad was backing the rebel attack, in which about 65 people were believed to have been killed.

Security forces cordoned off an area of Khartoum central on Monday, chasing a small group of suspected rebels into a building, witnesses said. "There's gun fire but it's one way. There's no exchange of fire," said one witness.

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

JEM 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.

Rebel leader Ibrahim called Turabi a "nuisance" and said he knew nothing about the attack.

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 and of backing rebels seeking their overthrow.

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 died and that 2.5 million have been made homeless in Darfur since mostly non-Arab rebels took up arms. The government says only 10,000 people have been killed.

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