Delegates from the former Angolan rebel movement Unita are in the capital Luanda to elect a new president to succeed Jonas Savimbi, who was killed in battle last year.
From guerrilla army to political party
The congress ends on Friday, and is intended to mark the movement's transition from guerrilla army to political party.
Following Mr Savimbi's death, Unita signed a peace treaty with the Angolan Government, bringing to an end a civil war that began at the time of independence from the Portuguese in 1975.
The two front-runners for president are Paulo Lukamba, better known as General Gato, or the Cat, and Isias Samakuva.
Both men have attracted Unita heavyweights to their camps, and some Unita supporters worry that a fierce leadership struggle could prove divisive in the long run.
Unita founded early 1960s by Jonas Savimbi
Engaged in civil war that lasted 27 years
One million Angolans dead
More than 100,000 Unita guerrillas demobilised
General Gato assumed the leadership of Unita after Mr Savimbi's death.
His move for the presidency has attracted most attention as he said specifically earlier this year that he would not seek election.
The BBC 's Southern Africa correspondent, Barnaby Phillips, says any divisions in the Unita leadership arising from the leadership contest would suit Angola's President, Jose Eduardo dos Santos.
The effective winner of Angola's civil war, President dos Santos retains a tight control over the country's vast oil revenues, and will be confident that he can defeat any Unita candidate as and when he decides to hold elections, our correspondent says.
Unita was founded in the 1960s by Mr Savimbi who led it through almost three decades of war until he was ambushed and killed by government soldiers in February last year.
He was known a powerful and tyrannical man who, with the help of apartheid South Africa and Ronald Reagan's America, turned Unita into a ruthless rebel army that controlled much of Angola.