KHARTOUM (Reuters) - Observers on Monday urged Sudan to extend voting in its first open elections in 24 years after thousands of ballots were cast incorrectly and polling faced serious delays in many areas of Africa's largest country.

The complex presidential, legislative and gubernatorial elections, which began on Sunday and were scheduled to last three days, had been hoped to transform Sudan from a nation emerging from decades of civil war to a democratic state.

But after a wave of opposition boycotts the vote now looks likely to confirm the 21-year rule of President Omar Hassan al-Bashir -- the only sitting head of state wanted for war crimes by the International Criminal Court, which alleges he was behind mass murder and rape in Darfur.

Sudanese election observers said polling stations in parts of the north and much of the south had experienced serious delays on Sunday and in some areas voting had not begun because ballot papers had not arrived.

"They may well really need to extend the period for these places where the materials haven't arrived," said al-Baqer Alafif, head of one of the largest Sudanese observer teams.

"In the south because the materials haven't arrived in many centres, some haven't even started voting yet," he added. He welcomed a call by the main south Sudan party for extended voting as many in the semi-autonomous south wasted hours searching for the correct voting stations.

"We want four on top of the three days ... in southern Sudan," said Samson Kwaje, campaign head of south Sudan's incumbent President Salva Kiir, who is also expected to be re-elected.

The elections and a plebiscite on independence for south Sudan next year are key parts of a 2005 peace deal that ended a two-decade-long civil war between Sudan's north and south.

'CLEARLY NOT READY'

While the first day of polling on Sunday ended with no reports of violence, Alafif said the National Elections Commission was "clearly not ready" to begin the elections on schedule and should have heeded calls from opposition parties and observers for a short delay to resolve logistical problems.

Problems were expected in the complex polls, with northerners using eight ballot papers and southerners grappling with 12, but the extent of the errors, election observers said, was very serious.

"It's quite clear that these violations were very serious ones," said Shamseddin Dawalbeit, the deputy head of Tamam, an alliance of more than 100 civil society groups working on the polls.

"These violations are being repeated systematically," he said.

Voting had not begun at all in White Nile state on Sunday, opposition representatives said, after ballots were printed incorrectly -- twice.

"The NEC did not have arrangements in place in Khartoum and the regions ... many places did not start voting until 4 p.m.," said observer al-Baqer Alafif. Voting for the state parliament in Port Sudan had stopped completely, he added.

Reuters witnesses saw many voting centres in Khartoum using the wrong ballots, with some not realising until hours into voting. Others were aware but refused to stop or change the ballots.

The NEC said there had been some "technical problems," but blamed British and South African printers for ballot errors and said voting had begun well across Sudan.

"(White Nile) will be compensated for by extra hours of voting in the next two days," a statement released late on Sunday said.

"Apart from these normal technical problems, the reports coming in from the south, Darfur and other regions of the country are that the voting process is going well," it added.

Sudan's main opposition parties had announced a boycott even before the polls began, citing widespread fraud and continuing conflict in Darfur. The NEC rejected their accusations that it was biased towards Bashir's National Congress Party.