By Fatma Naib in Khartoum
International observers, including former US president Jimmy Carter, have arrived in Sudan to monitor the referendum which could see the south of the country separated from the north.
The European Union will have 104 observers and experts present for the vote. The Carter Center has deployed 75 observers. China, which has large investments in Sudan's oil sector, is also sending observers.
Carter himself arrived in Sudan on Friday along with his wife. He was expected to meet President Omar al-Bashir, African Union officials and leaders from north and south during his visit.
The Carter Center has had observers deployed in Sudan for more than two years and were monitoring the presidential election in April last year. Carter said Sunday's referendum in the south is very important for the centre.
"We have been waiting for the referendum for six years, since the CPA (Comprehensive Peace Agreement) was signed.
"The Carter Center has 10,000 volunteers working for our centre in the south. We have 120 employees in addition to the observers who are here for the referendum. What happens in Sudan is very important to us in terms of its future prosperity, health and peace for all the people in north and south."
'Accept the result'
Asked what his message was to people in the north and the south, Carter said: "Just accept the decision of the southerners in the referendum peacefully, no matter whether it is to stay part of Sudan or to form a new nation.
"Then after that, devote full time to the complete implementation of the Comprehensive Peace Agreement provisions dealing with Abyei, dealing with the borders and divisions of wealth, in particular oil."
Carter expressed optimism that the dispute over the border region Abyei would be resolved peacefully.
"I am sure that nobody in north or south wants to go back to war. That is the biggest factor in encouraging everybody in resolving things peacefully," he said.
John Kerry, a US senator, also arrived in Sudan on Friday and said he had listened to a number of different views from university teachers, politicians, and party leaders.
"In the US we believe in listening to many voices and making sure that everyone has an opportunity to be heard and that is important in this process to move forward in Sudan," Kerry said.
He added that the referendum is an enormous opportunity for Sudan as a whole, for the north and the south to re-engage with the world, and to change its relationship in particular with the US.
"History can be written by the people of Sudan."