More than 60 per cent of those registered must vote in Sudan's referendum for the result to be valid. [AFP]

Salva Kiir, the president of South Sudan, has appealed to southerners to sign up en masse as registration kicked off for a January referendum that may lead to the partition of Africa's largest country.

After a three-month delay, the registration process began on Monday, with an air of celebration in the south, while it was marked with indifference in the mainly Muslim north, where stores were closed for the Eid Al-Adha holiday.

Kiir appeared early at a registration centre in the southern capital of Juba outside a memorial to John Garang, who led the mostly Christian south to a 2005 peace deal that ended a 22-year war with the north before he died in a helicopter crash.

"A referendum happens only once. People must come out en masse otherwise it would mean people fought and died for nothing," the leader of the autonomous south said, as hundreds lined up outside a registration centre.

The January 9 plebiscite on southern independence ends a peace process that began with so much hope in 2005 when Sudan's north and south ended Africa's longest civil war and embarked on a six-year process to make unity an attractive option to southerners.

Much of Sudan's modest 470,000 barrels per day of oil lies in the south.

Slow registration

Al Jazeera's Haru Mutasa, reporting from Juba, said that many there are "excited about the possibility of change" but that it was still tough to get people to register.

"Some would rather spend the day putting food on the table than wait in long queues - and there are other challenges" she said.

"Decades of civil war disrupted life for people in Southern Sudan," our correspondent said.

"Many of them don't even have identity documents, and they don't know if they'll be allowed to register."

In the north, registration centres were empty as many southerners who live in the Sudanese capital of Khartoum made the trip south to enrol.

Al-Tayib Zain al-Abdin, a professor of political science at the University of Khartoum in Sudan, told Al Jazeera that the registration there is "very weak" for various reasons.

The Eid holidays, he said, are one factor.

"The Eid definitely is a handicap. And in fact, when they set the dates first, I think they have forgotten about the Eid, so now they have to face it,"al-Abdin said.

"Secondly, some southerners in displaced camps were told that they should not register in the north," al-Abdin said, adding that these southerners were told to register and vote in the south.

He said that some southerners were told that if they could not register in the south, then they shouldn't register at all.

About five million south Sudanese are eligible to register for the vote, including an estimated 500,000 to two million who live abroad, according to UN estimates.

"We expect the process to start very slowly but they will catch up and they will build the momentum after a couple of days and things should run more smoothly," Denis Kadima, director of the UN Integrated Referendum and Electoral Division, said.