Zambia's long-time opposition leader Michael Sata has been sworn in as president after a decisive victory over the incumbent in a presidential poll this week.

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

After taking office, Mr. Sata promised to help the nation's poor and to fight corruption.

President Sata also assured foreign investors that they are welcome in his country, Africa's biggest copper producer, but said they must improve conditions for their Zambian employees.

In the past, Mr. Sata has criticized China, which has invested heavily in Zambia's mining industry.

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

The electoral commission said 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.”

The victory by Mr. Sata and his Patriotic Front ends two decades of rule by the Movement for Multiparty Democracy, which came to power in Zambia's first multi-party elections in 1991.

On Friday, global leaders congratulated the people of Zambia for the country's democratic elections and smooth transition of power.

President Barack Obama said the United States looks forward to working with President Sata, and he thanked former president Banda for contributing to Zambia's democratic development.

U.N. Secretary-General Ban Ki-moon said Zambians have shown Africa and the world that when democracy is allowed to flourish, change can be brought about peacefully.

Analysts do not expect Mr. Sata to make major policy changes, although his Patriotic Front has promised to re-instate a 25 percent windfall tax on mining revenues, which Mr. Banda's party abolished in 2009.

Mr. Sata had 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.

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.

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