President Joseph Kabila has been sworn in as Democratic Republic of Congo's head of state for a period of two years before elections are held.
Hema/Lendu clashes have led to thousands of deaths
A new transitional government should be formed soon, including representatives of rebel groups who control eastern DR Congo but they were not present at Monday's ceremony in the capital, Kinshasa.
President Kabila assumed the full powers of an interim head of state, including ensuring that all parties abide by the peace pact finalised last week in South Africa.
Meanwhile, peace talks in the north-east resumed on Monday, despite claims that ethnic fighters massacred up to 1,000 people last week.
Survivors told UN human rights investigators that the killings took place on Thursday morning and showed them the sites of 20 mass graves near Drodro, about 80 kilometres (50 miles) from the regional capital Bunia.
UN investigators returned to the scene of the massacre on Monday vowing to uncover the truth.
"We want to find out what happened, why it happened, who did that," said the spokesman for the UN force in DR Congo (Monuc), Hamadoun Toure.
The peace talks, known as the Ituri Pacification Commission, have grouped together representatives of the different communities and militia groups that have fought for control of the gold-rich Ituri district for the past four years.
Over the weekend, President Kabila promulgated the new constitution agreed in the South African resort of Sun City.
But the ethnic clashes between the Hema and Lendu ethnic militias in Ituri remain a potential stumbling block for the peace process.
Representatives of the Hema community, who were targeted this time, had threatened to pull out of the talks, which are a spin-off of the main peace accords.
On Monday morning, a Ugandan army spokesman denied any involvement in the massacre, saying his troops had been at least 15 km away.
He said Ugandan troops would stay in the area "for the protection of the people" even though there is a 24 April deadline for their withdrawal.
The leader of local rebel group Union of Congolese Patriots (UPC), Thomas Lubanga, had accused the Ugandans of infiltrating the militia responsible for the alleged massacre, which took place last Thursday.
According to lists compiled by leaders, 966 people died in three hours of blood-letting.
Local groups said the clashes that started out as a simple land dispute between pastoralists and farmers have killed more than 50,000 people and displaced a further 500,000.