Sudan's election commission has extended voting for two days after a series of delays and technical problems were reported at polling stations across the country.
The elections were scheduled to run through Tuesday, but Election Commission spokesman Salah Habib says the final day will now be Thursday.
Officials of southern Sudan's dominant party, the Sudan People's Liberation Movement, had called for an extension after the first day of voting on Sunday.
The balloting was calm but election monitors said many polling stations opened late, used incorrect ballots, and had trouble finding people on the register.
Problems seemed to be worse in the south, where some polling stations had yet to receive voting materials by Monday morning.
In many areas, voting has been smooth, but observers say people are often confused by the multiple ballots for president, parliament, and state and regional races.
These are Sudan's first multi-party elections since 1986, and for many people, their first time voting.
Several parties are either fully or partially boycotting the elections, saying President Omar al-Bashir and the ruling National Congress Party have planned to rig the results.
On Sunday, Sudan's national elections commission said rigging was "impossible" but later acknowledged some technical problems.
President Bashir is almost certain to win re-election, after two of his main challengers, Yasir Arman from the SPLM and former Prime Minister Sadiq al-Mahdi, pulled out of the race last week.
The elections are a key part of the 2005 peace deal that ended Sudan's north-south civil war. That deal calls for a referendum next year in which southern Sudan, which now has partial autonomy, will decide whether to become fully independent.
President Bashir has ruled Sudan since a 1989 coup. He is wanted by the International Criminal Court for alleged war crimes committed in the western region of Darfur. His government has been fighting rebels there since 2003.
Some information for this report was provided by AFP and Reuters.