Unidentified gunmen have launched two attacks on peacekeepers in Sudan's troubled Darfur region, losing four of their own men in one of them, the U.N.-African Union mission said on Friday.

The U.N.-AU mission in Darfur deployed to the region in 2007, four years after ethnic insurgents mounted a campaign against the Arab-dominated government of Omar al-Bashir, complaining of their marginalization.

Sudanese President Omar al-Bashir Friday labeled foreign critics of last week's general elections, which he is widely expected to win, as "colonialist parties" and said they would have no effect on the polls.

Britain, Norway and the United States had issued a joint statement criticizing Khartoum for failing "to create a free, fair, and conducive elections environment."

A South Sudanese opposition leader who has spoken out against both sides in the civil war said Friday he appeared to have been placed under house arrest.

Opposition politician Lam Akol said his home in the capital Juba, which is under the control of the government loyal to President Salva Kiir, was surrounded by security personnel during the night.

Sudan summoned the British, Norwegian and U.S. ambassadors Tuesday over a statement criticizing last week's elections, which are expected to give President Omar al-Bashir another term in office.

The three countries said Monday they regretted Khartoum's "failure to create a free, fair and conducive elections environment."

Sudan's National Intelligence and Security Service (NISS) has detained an opposition activist who campaigned on behalf of a colleague who was taken into custody during last week's elections, his party said Tuesday.

"Jalal Mustafa, a party leader, is still in custody after he was taken yesterday (Monday) from the home of Sandra Kaduda," Sudanese Congress Party Secretary General Abdel Gayoum Awad told AFP.

Sudan on Wednesday extended nationwide elections by one day after a low turnout that the opposition said reflected apathy towards a vote President Omar al-Bashir is widely expected to win.

The 71-year-old career soldier, indicted by the International Criminal Court for alleged war crimes, is seeking to extend his quarter-century rule virtually unopposed.

Sudanese rebels have attacked several polling stations in war-torn South Kordofan state during nationwide elections widely expected to extend President Omar al-Bashir's 25-year rule.

Bashir, 71, who is wanted for alleged war crimes in the western region of Darfur, also faces rebels in South Kordofan and Blue Nile states.

Battling division and government repression, Sudan's opposition has launched a sit-in against elections expected to extend President Omar al-Bashir's rule, but it has been met with almost as much indifference as the vote itself.

Only about 80 people gathered at the headquarters of the country's oldest political party, the Umma, in Khartoum's twin city of Omdurman as night fell on the first day of the elections on Monday.

Sudan was voting Monday in elections boycotted by the mainstream opposition that are expected to extend the quarter-century rule of President Omar al-Bashir, who is wanted on war crimes charges.

With 15 little-known candidates running against him, 71-year-old Bashir is virtually unchallenged in the vote, which has already been criticized by the international community.

Sudan's elections are of little concern to the latest arrivals at Darfur's Zamzam camp, forced to flee their homes by fighting this year between rebels and President Omar al-Bashir's forces.

Indicted by the International Criminal Court over alleged war crimes in the western region, Bashir is widely expected to extend his rule in polls starting on Monday.