(Corrects to show Mariam is al-Madhi's daughter, not granddaughter, in paragraph 4)
* Opposition party calls off talks over arrest
* Two newly added charges could lead to al-Mahdi's execution
* His party calls on its supporters to protest
By Khalid Abdel Aziz
KHARTOUM, May 17 (Reuters) - Sudanese opposition leader Sadiq al-Mahdi was arrested on Saturday on charges that could lead to the death penalty, a government official said, a move that could hurt efforts to ease political tensions before elections due next year.
Al-Mahdi, a former prime minister in Sudan's last elected civilian government, is the head of the Umma Party, the most prominent party opposing President Omar Hassan al-Bashir, who ousted him in 1989.
The public prosecutor had in the past days already opened an investigation into accusations that he insulted state security forces over a surge in violence in the troubled Darfur region.
Al-Mahdi's daughter Mariam said his lawyer had briefed her on the accusations. "He said charges of halting the constitutional system and inciting hatred against the state were added and their penalties range from life in prison to death," she told Reuters.
A government official, who declined to be named, confirmed al-Mahdi's arrest and the possible punishment he could face and said the investigations into him would start on Sunday.
In response to the arrest, the Umma party cancelled national dialogue talks called by the president, that were meant to ease tension among Sudan's political parties ahead of parliamentary and presidential polls due next year, especially over the handling of Darfur. No firm date had been set for the talks.
It called on supporters to protest against the detention.
Bashir has been working to shore up his power in the face of an economic crisis since South Sudan seceded in 2011, taking with it three-quarters of the once unified nation's oil output; protests against him and the violence in Darfur.
He has remained in office despite the sporadic rebellions, U.S. trade sanctions, the economic crisis, an attempted coup and an indictment from the International Criminal Court on charges of masterminding genocide and other war crimes in Darfur.
Al-Mahdi's personal assistant and office manager, Mohamed Zaki, told Reuters two security officers came to al-Mahdi's office, in his home in Khartoum, at 8:45 p.m. and arrested him without giving any reason.
"The Umma Party has decided to stop the dialogue with the National Congress Party, the ruling party, and asks for the release of Sadiq al-Mahdi," Umma Secretary-General Sarah Naqdallah told reporters at al-Madhi's house.
"By arresting Sadiq al-Mahdi the regime has gone back on all of its promises about dialogue and has returned to square one ... (the Umma Party) calls on all of its grass roots supporters to express rejection of this step in the capital and the provinces in a peaceful fashion."
One analyst said the president's proposal for the national talks may have only been a stalling tactic in the first place.
"May be this step points to the ruling party not being serious about the dialogue and that it was a tactical question only ... How can you arrest the main side with which you want to hold a dialogue?" political analyst Khaled al-Tujani said.
Western diplomats and Sudanese security sources estimate that thousands have been killed in clashes between militias supporting and opposing the government in Darfur since March.
Opposition leaders have accused government forces of attacking civilians in the region. The U.S. ambassador to the United Nations has also accused Khartoum of obstructing the joint U.N.-African Union peace keeping force, known as UNAMID.
The United States in March said civilians were being "terrorised, displaced, and killed" despite the presence of one of the world's biggest peace keeping missions.
Law and order has collapsed in much of the huge region, where mainly African tribes took up arms in 2003 against the Arab-led government in Khartoum, which they accused of discriminating against them.
UNAMID has been deployed since 2007. The conflict in Darfur has killed as many as 300,000 people and displaced around 2 million, according to the United Nations. (Writing by Shadia Nasralla and Yasmine Saleh; Editing by Michael Georgy, Alison Williams and Lisa Shumaker)