Zambia's long-time opposition leader Michael Sata has been sworn in as president after unseating the incumbent president in a tight race.

The 74-year-old populist figure took the oath of office at the Supreme Court in the capital, Lusaka, on Friday.

After the swearing in, Mr. Sata vowed to help the nation's poor and assured foreign investors that they are welcome in his country, Africa's biggest copper producer.

President Sata appeared to be downplaying previous criticism of China, which has invested heavily in the Zambia's mining industry.

Mr. Sata was declared the winner of Tuesday's presidential poll just after midnight on Friday, prompting supporters to take to the streets in celebration.

The electoral commission announced that Mr. Sata defeated incumbent President Rupiah Banda with 43 percent of the vote compared to Mr. Banda's 36 percent.

In a tearful news conference, President Banda conceded defeat, saying that “the people of Zambia have spoken and we must all listen.” He urged supporters to reject acts of retribution, saying “now is not the time for violence.”

There were scattered reports of violence across the country this week amid frustrations over the slow release of election results. Many opposition members said they feared the delay would allow time for the commission to skew the results in Mr. Banda's favor.

Ahead of the poll, Mr. Sata, known as “King Cobra” for his sharp wit and fiery speeches, accused the electoral commission of planning to rig the outcome using pre-marked ballots – an allegation the commission denied.

European Union election observers have said the elections were “generally well administered,” but unequal access to resources meant there was not a “level playing field” for campaigning.

Specifically, observers criticized state-owned media for failing to meet “even their minimal obligations as public service media,” saying state news programming lacked balance in its coverage of the campaign.

Analysts do not expect many major policy changes when Mr. Sata takes office, though his Patriotic Front party has promised to re-instate a 25 percent windfall tax on mining revenues that Banda's party abolished in 2009.

Mr. Sata repeatedly accused Mr. Banda of tolerating corruption and not doing enough to ensure that more Zambians share in the wealth of the country's copper reserves.

The two men are long-time rivals. In 2008, Mr. Banda defeated Mr. Sata by two percentage points in a special election triggered by the death of late President Levy Mwanawasa, who died of a stroke.

Mr. Banda's Movement for Multiparty Democracy had ruled the country since former president Frederick Chiluba defeated independence leader Kenneth Kaunda in the country's first democratic elections held in 1991.

Formerly known as Northern Rhodesia, Zambia declared independence from Britain in 1964.