KHARTOUM (Reuters) - An international think tank has accused Sudan's ruling party of trying to rig elections in wartorn Darfur region, as the country prepares for its first multi-party polls in 24 years.

The International Crisis Group said voter registers for the April 11-18 polls had been manipulated, constituencies based on a flawed 2008 census and the election commission staffed with too many pro-government officials, in a report released late on Tuesday.

Darfur has 19 percent of the population of Sudan. Incumbent President Omar Hassan al-Bashir's dominant National Congress Party can expect little support in south Sudan, which has slightly more people.

"Winning big in Darfur is thus central to the NCP's hopes of capturing enough votes in northern Sudan to ensure its continued national dominance," the report said.

"According to national and international observers alike, many groups targeted in the conflict, especially IDPs (internally displaced people), were unable to register (to vote) or refused to do so," the report said.

"In many instances, people were deliberately denied sufficient time and information, while teams worked hard in remote areas to register nomads who support the government."

The United Nations estimates the conflict which began in North Darfur in 2003 has driven more than 2.5 million from their homes, with some 200,000 refugees in neighbouring Chad. It estimates 300,000 have died in the ensuing humanitarian crisis.

Bashir is wanted by the International Criminal Court for war crimes in Darfur.

Most of those displaced fled North Darfur to camps in the west and south, the United Nations has said. But the 2008 census found the population had increased in North Darfur and decreased in West Darfur.

"The impact of the census is very significant," the ICG said. "Western Darfur (was) allocated only fifteen seats in the national assembly compared to 24 for Northern Darfur."

"The rebel-held Jebel Marra massif in Southern Darfur, with an estimated population of 1 million but only 35,000 according to the 2008 census, has no seats," it added.

ICG said the mid-ranks of the National Elections Commission had been filled with NCP supporters, an allegation the commission has denied.

The NCP was not immediately available to comment on the report. It has previously dismissed similar opposition comments.

The report argued the international community should award no legitimacy to whoever won the polls.

"Since the vote will impose illegitimate officials through rigged polls, they (Darfuris) will be left with little or no hope of a peaceful change in the status quo, and many can be expected to look to rebel groups to fight and win back their lost rights and lands," it said.

Sudan's opposition groups have said the conflict and emergency law in Darfur have hindered their campaigns there and some have asked for elections to be delayed.