Al-Bashir, who seized power with support from conservative Muslim groups, is facing off against 11 other hopefuls.
They include Fatima Ahmed Abdelmahmud, the first woman ever to aspire to the presidency, and Sadiq al-Mahdi, the two-time former prime minister whom al-Bashir ousted.
Al-Bashir is the world's first sitting president facing an international arrest warrant from the International Criminal Court (ICC) on charges of alleged crimes against humanity in Sudan's western Darfur region.
The UN says up to 300,000 people have died and 2.7 million fled their homes since the ethnic minority rebels in Darfur first rose up against the Arab-dominated Khartoum government in February 2003.
But the Sudanese government disputes the death toll, saying around 10,000 people have died.
Al-Bashir's main challengers are Yasser Arman, a secular Muslim from north Sudan representing the former rebel Sudan People's Liberation Movement, and al-Mahdi, the former prime minister from the influential Umma Party.
Arman, 49, is counting on the solid support of south Sudan, while the 74-year-old Mahdi's main support base is in the north.
But in a country without opinion polls and which has not held real elections in decades, the outcome of the polls is wide open.
The opposition fear al-Bashir will use the levers of power, including the security forces, to win the vote.
Rallies have been banned, but the opposition plans to test the waters during campaigning to try and stage one in Omdurman, the twin city of Khartoum, Sudan's capital.
Sudan has emerged from a devastating 22-year civil war that pitted the dominant Muslim and Arab northern Sudanese against the largely non-Muslim, non-Arab southerners.
A comprehensive peace agreement was signed with the south in 2005, ending the conflict.
But the most basic services are lacking in much of the remote and underdeveloped regions of Sudan, which has 41 million people.