Violence in the disputed region of Abyei has killed at least 30 people along Sudan's north-south divide, as voters flocked to polling stations Monday officials say.

Long lines formed outside polling stations, which opened Sunday, as thousands of people waited to cast a ballot in the referendum that will determine whether the south will split from the north.

CBC's Carolyn Dunn said from Juba, the capital of Southern Sudan, that many people waited for as long as eight hours in the blistering heat before they could cast their ballot.

Rachel Anai said she would have waited even longer for the chance to vote for an independent Southern Sudan.

"It is our vision we have been struggling for ... a very long time," she said Sunday. "When you are a citizen, you don't like to be second-class in your own country."

A voter has her fingerprint taken by a poll worker before voting in Southern Sudan on Sunday. (Paul Banks/UNMIS/Reuters)

The referendum is the result of a 2005 peace agreement that ended decades of civil war between the mostly Muslim north and the Christian and animist south.

The seven days of voting are widely expected to produce an overwhelming vote for independence, and Sudanese President Omar al-Bashir has said he will let the oil-rich south secede peacefully.

International observers, including former U.S. president Jimmy Carter, are in Sudan to monitor the vote.

Voting is scheduled to run until Jan. 15 and full results aren't expected until February. Independence wouldn't be finalized until July and negotiations on issues like oil rights, water rights and borders are expected to continue in the months ahead.

There have also been concerns about how Sudan's debt will be shared, though Carter told reporters Monday that the north has said it will take on the country's $38-billion US debt if the south votes to secede.

At least 60 per cent of the roughly 3.9 million people registered to vote must cast ballots for the results of the vote to be valid.

Clashes in Abyei

Meanwhile, more than 30 people have been killed in recent days in clashes in Abyei.

The clashes were reportedly between the Misseriya, an Arab tribe that moves its cattle herds through Abyei, and the Dinka Ngok, who have ties to the south.

Reports say more than 30 people have died, but the exact number is not clear. Each side blames the other for starting the violence.

The Abyei region is not taking part in the current referendum.

The region was promised a separate vote to determine whether to join the north or the south if the south separates, but that vote was postponed indefinitely because of instability in the region and concerns over who would be eligible to vote.

With files from CBC's Carolyn Dunn and The Associated Press