By Opheera McDoom

KHARTOUM, May 13 (Reuters) - Human Rights Watch voiced concern on Tuesday at mass arrests in Khartoum after an attack on Sudan's capital by Darfur rebels and said it feared some people had been tortured or killed.

Authorities were shaken by the attack, 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 country.

New York-based Human Rights Watch urged the government to quickly try or release those arrested in the aftermath.

"The whereabouts of the majority of those arrested are unknown," it said in a statement.

"Human Rights Watch received unconfirmed reports that some of those arrested have been tortured and that at least two people have been summarily executed in public."

The SUNA state news agency said at least 300 people had been arrested by Sunday, but many more have been arrested since.

President Omar Hassan al-Bashir said all those connected to the attack by the Justice and Equality Movement (JEM) would get a fair trial. Darfur rebels took up arms in 2003 complaining of discrimination against the largely non-Arab region.

"The President... affirmed the provision of a fair trial with the right to defence for all those involved in the brutal attack on Omdurman," SUNA said.

Darfuri human rights lawyer Saleh Mahmoud Osman said his brother was arrested on Sunday and he did not know where he was.

"On public transport, even on the streets, people are identified and treated openly in a harsh way because of their colour or because they look Darfuri," he told Reuters, estimating the number of arrests in the hundreds but not thousands.

MOST PARTIES UNITED

Almost all Sudan's main political parties have condemned the attack on the western suburb of Omdurman, on the opposite bank of the Nile river to central Khartoum. At least 65 combatants died in the fighting and a Sudanese security source said around 100 civilians were also killed or wounded.

Bashir held a rare meeting with opposition Umma Party leader and former Prime Minister Sadig al-Mahdi after the attack.

But security forces detained opposition Islamist Hassan al-Turabi for over 12 hours on Monday for questioning. He and several other leaders of his Popular Congress Party were released without charge.

JEM rebel leader Khalil Ibrahim was once allied to Turabi, but both deny any links now. Turabi's party has said it will have to meet before it issues a formal statement on the attack.

Burnt out vehicles and broken glass littered Omdurman's streets, recalling the destruction in decades of conflicts that have scarred the south, Darfur and the east of Sudan but never the capital, which is enjoying the profits of an oil boom.

JEM's forces have retreated to the neighbouring Kordofan province since the fighting at the weekend. Ibrahim, who has a price of $125,000 on his head, has said he will keep attacking Khartoum until Bashir falls.

Sudan has blamed neighbouring Chad for the attack and cut diplomatic relations. Chad on Monday closed its border with Sudan but denied any link to the assault.

Darfur's five year conflict has claimed an estimated 200,000 lives and driven more than 2.5 million from their homes. Rebels from Sudan's multiple regional wars all feel the central government has marginalised their areas.

Khartoum blames the Western media for exaggerating the conflict and puts the death toll at 10,000.

(Editing by Matthew Tostevin)