He also vowed to clear his Cabinet of faction leaders who had aided the United States in ousting the Taliban three years ago but who have proved to be deadweights in office.
Karzai, who is to be inaugurated in early December, also promised to accelerate the rebuilding of the economy, devastated by war and drought, with the goal of doubling the income of ordinary Afghans by 2009.
"The Afghan people have placed their trust in us, for which we are very grateful," Karzai said at his Presidential Palace, flanked by his two running mates. "It will be hard to live up to but we will do our best."
Election officials declared Karzai the winner Wednesday after three weeks of laborious counting and allegations of voter fraud and polling errors in the election process brought by his opponents. Eighteen candidates ran for the presidency in the country's twice-delayed, historic Oct 9 election, which was largely peaceful.
A panel of foreign experts that monitored the allegations said it found a string of irregularities, including ballot-stuffing, "But they could not have materially affected the overall result," the panel said.
Yunus Qanooni, who finished second with 16 percent compared to Karzai's 55 percent, accepted the result just hours before Karzai made his televised acceptance speech. Ethnic Hazara chieftain Mohammed Mohaqeq and ethnic Uzbek strongman Abdul Rashid Dostum followed suit.
Casting a shadow over Karzai's victory, and highlighting the country's insecurity, has been the abduction of three foreign election workers last week.
Jaish-al Muslimeen, a splinter group of the Taliban, has taken responsibility for the kidnappings, but officials doubt the little-known group could have done it without the help of local militiamen.
The group released a videotape of the hostages Sunday and has demanded the withdrawal of British troops and the United Nations from Afghanistan in return for the hostages' lives.
On Thursday, the group said that talks on their demands, including the release of Taliban prisoners, had broken down, and they would decide Friday whether to kill the trio.
"The government said they would think about releasing those Taliban held in Afghan prisons and discuss the prisoners held by the Americans," spokesman Syed Khaled said.
UN spokesman Manoel de Almeida e Silva thanked the Afghan government Thursday for its efforts to free the hostages. He said the world body was "doing all it can" in support, but gave no details.
The abductions were the first kidnappings of foreigners in Kabul since the Taliban was ousted in 2001, and sparked concern militants were copying the tactics of their Iraqi counterparts.
Electoral Board Chairman, Zakim Shah appealed for the swift release of the victims "to put an end to this suffering during what should be a time of national celebration."