* Congress Party leader arrested after critical speech
* Blamed government for recent surge of violence in Darfur
* Charges include undermining constitutional order
* Umma Party leader arrested mid-May on similar charges
KHARTOUM, June 8 Sudanese authorities arrested on Sunday the head of Sudan's opposition Congress Party, a party official said, weeks after the arrest of another opposition leader, Sadiq al-Mahdi, sparked violent anti-government protests.
The arrests are likely to hurt national dialogue talks that were called by President Omar Hassan al-Bashir in order to ease tension among Sudan's political parties ahead of parliamentary and presidential polls due next year.
Opposition parties in Sudan have voiced concern about the government's mismanagement of Darfur, accusing it of killing civilians during a recent surge of violence in the region.
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.
"The police forces detained our party head Ibrahim al-Sheikh for participating in a political lecture yesterday night," the Congress Party's General Secretary Abdel Kayoum Awad told Reuters.
Awad said the prosecution has charged Sheikh with undermining the constitutional order by spreading "harmful lies" after he blamed the government for the recent violence in Darfur in a speech on Saturday.
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.
The conflict killed 300,000 people and displaced around 2 million, according to the United Nations. Bashir has a pending arrest warrant against him from the International Criminal Court on charges of masterminding genocide and other war crimes in Darfur.
The charge of undermining the constitutional order, which Mahdi has also been accused of, can lead to the death penalty, a government official said previously.
Mahdi, a former prime minister, was arrested on May 17. He was also charged with insulting the state security forces by accusing them of participating in the Darfur unrest.
In response to the arrest, Mahdi's Umma party withdrew from the national dialogue talks.
Since then hundreds of Sudanese have gathered for protests after Friday prayers, calling for Mahdi's release and chanting "The people want to change the regime" and "Freedom, peace and justice". (Reporting by Khalid Abdel Aziz in Khartoum, Omar Fahmy in Cairo and Writing by Yasmine Saleh in Cairo; Editing by Raissa Kasolowsky)