Khartoum - The arrest of Sudan's former prime minister and leading opposition figure Sadiq al-Mahdi undermines talks aimed at finding a way out of the country's multiple crises, observers said on Sunday.
Mahdi was arrested for alleged treason by agents of the National Intelligence and Security Service (NISS) from his home on Saturday night, officials of his Umma Party said.
Prosecutors questioned him on Sunday at the Khartoum-area Kober Prison where he is being held, Umma's secretary general Sara Najdallah, told reporters.
The arrest came after Mahdi reportedly accused a counter-insurgency unit, the Rapid Support Forces (RSF), of rape and other abuses of civilians in Darfur.
"We are deeply concerned. This isn't the way to do national dialogue," a British embassy spokesman said.
Magdi El Gizouli, a fellow at the Rift Valley Institute, said Mahdi's detention reflects a power play by elements of the state security service who "don't care" about the impact on the dialogue initiated by President Omar al-Bashir.
"The political system in Sudan is the rule of the military and the security," Gizouli told AFP.
Late Sunday NISS, which has authority over the RSF, announced in a notice to local journalists that the unit has been ordered deployed around the capital Khartoum.
Umma and the Popular Congress opposition of veteran Islamist Hassan al-Turabi were key targets of Bashir's dialogue which, a senior opposition figure has said, might have led to a coalition government.
Following Mahdi's arrest, Najdallah said her party was withdrawing from the dialogue and accused the government of going back to "square one."
She announced an "alert" for Umma supporters to peacefully oppose their leader's arrest.
Hundreds of them gathered on Sunday at Umma's Khartoum-area headquarters, an AFP reporter said.
They carried signs saying: "No dialogue with state security" and "Sadiq al-Mahdi's opinion is the opinion of the people."
'Double-headed' ruling party
The Bashir regime has faced mounting challenges since the separation of South Sudan three years ago.
In January the president appealed for a broad national political dialogue and "renaissance" focused on peace in the war-torn, impoverished country.
But there have been warnings that his ruling National Congress Party (NCP) was not fully behind the reforms, which led to a tenuous political opening that has allowed opposition rallies to occur without disruption by security forces.
"I think there's a sort of double-headed NCP now," Gizouli said.
An opposition politician, who asked not to be identified, also referred to purported divisions in the NCP and said: "By arresting Sadiq, that would be a very good way of stopping the dialogue."
There are also suggestions that the Islamist Turabi would not want to share power with Mahdi if the dialogue produced a new government, and that Mahdi himself could benefit from "a revival of his image" by being arrested, the politician said.
Mahdi, a political fixture since the 1960s, is now regarded with suspicion by many Sudanese, partly because one of his sons is a Bashir adviser and another is a relatively junior NISS officer.
Opposition activist Asma Mahmoud Muhammad Taha said it never seems clear whether Mahdi is with the government or its opponents.
But Gizouli said Mahdi, a descendant of Sudan's legendary Islamic reformer known as "the Mahdi", remains a prominent religious leader and authority figure.
Newspapers reported on Tuesday that the security service filed a criminal complaint against Mahdi for his public comments about the troops.
Commanders of the RSF then denied at a news conference that their force had looted, raped or committed arson.
They are keen to present themselves as regular forces, "not vagabonds", to avoid the possibility of being disowned later by the regime, Gizouli said.
"Whatever changes happen in Khartoum, the first victims of such change would be the security establishment," he said.