A new era is beginning, President Joseph Kabila has told an audience in the Democratic Republic of Congo's capital shortly after his inauguration.
He told heads of state and an excited crowd that he would abide by "the trilogy of good governance, democracy and respect for human rights".
He is DR Congo's first freely elected leader in 40 years having won a tense run-off presidential poll in October.
The 35-year-old took power in 2001 after his father was assassinated.
Defeated presidential candidate and ex-rebel leader Jean-Pierre Bemba missed the ceremony.
"A new page is opening up before us. I can see the Congo of tomorrow carrying the hopes of a renascent Africa at the dawn of this century with its great challenges," Mr Kabila told the audience.
Thousands of guests are taking shelter from the hot sun
President Kabila also saluted the "remarkable solidarity" of the international community during the country's political transition, which has seen 17,000 United Nations peacekeepers implementing a 2002 peace deal following a five-year war.
Thousands of guests were in the garden of State House just near the huge and powerful Congo river for the ceremony.
Many people were shading themselves under umbrellas in the national colours of blue, red and yellow.
Several African leaders attended the event including South Africa's President Thabo Mbeki and the Angolan leader, Jose Eduardo dos Santos.
President Robert Mugabe of Zimbabwe did not attend - despite being a close ally of Joseph Kabila's father, Laurent.
Last month, the Supreme Court rejected a legal challenge by Mr Bemba, who claimed the run-off vote was rigged.
The poll was intended to bring a new era of stability after years of war and its successful conclusion was hailed as a miracle.
DR CONGO WAR
1998 - 2002
Armies from several countries and many rebel groups
2003: Rebels join unity government
East remains unstable
17,000 UN peacekeepers
About four million people died during the conflict in the east of the country that pitted government forces, supported by Angola, Namibia and Zimbabwe, against rebels backed by Uganda and Rwanda.
The challenges facing President Kabila remain huge.
Violence has continued involving small militia groups in the east who do not accept control from Kinshasa, the capital, which lies some 1,500km to the west away across vast tracts of forest.
On Tuesday, Ugandan military officials said at least 12,000 people from eastern DR Congo had crossed into the country to flee fighting between the Congolese army and forces loyal to dissident general Laurent Nkunda.
Ugandan officials say the fighting has now stopped near the border and national army is now back in control of the border posts.
Mr Nkunda left the army and launched his own low-level rebellion following the 2002 ceasefire, saying the country's transition to democracy was flawed and excluded the minority Tutsi community.
Mr Kabila won 58.05% of the vote compared to Mr Bemba's 41.9%, according to the Independent Electoral Commission.
US election observer group the Carter Center said there was evidence of vote tampering on both sides.
But it said neither candidate benefited significantly over the other.
The results showed a regional divide, with Mr Bemba gaining most votes in the Lingala-speaking west, including Kinshasa, while Mr Kabila won by a landslide in the Swahili-speaking east.
Under the peace deal, former rebels were supposed to be integrated into the army, but progress has been slow and the three former rebel leaders who are vice-presidents have retained large personal security forces.