Mugabe, Africa's oldest leader at 89, took the oath on Thursday for his seventh five-year term as Zimbabwe's lone ruler after independence from Britain: first as prime minister from 1980 to 1987 and ever since as president. He has told critics of his re-election to "go hang" and vowed to press ahead with nationalist policies forcing foreign firms to turn over majority stakes to black Zimbabweans.

Mugabe used more mild terms during Thursday's swearing-in ceremony, pledging to Chief Justice Godfrey Chidyausiku "to observe, uphold and defend the constitution of Zimbabwe."

According to the state-run Herald newspaper, the leaders of several African countries attended the swearing-in.

However, the Movement for a Democratic Change (MDC) - led by Tsvangirai, Mugabe's opponent in the last election - called the vote "null and void" and boycotted the swearing-in ceremony, held at a football stadium in Harare.

'A farce'

The opposition leader Tsvangirai had launched a legal challenge to the election results, alleging that the polls had excluded up to a million eligible voters and demanding the release of electoral rolls. He condemned the polls as "a farce," but, in a surprise U-turn last week, Tsvangirai and the MDC withdrew the petition, saying that they would not get a fair hearing.

Mugabe's Zanu-PF party also won a two-thirds majority in parliament in the July 31 vote, according to Zimbabwe's electoral commission. The result ended a power-sharing deal between Zanu-PF and the MDC.

The African Union has backed the election results on the grounds that problems in the election have not proved serious enough to declare the entire process invalid. Other observers, however, have cited severe flaws, in areas such as voter registration.

Britain demands investigation

Britain has called the result of Zimbabwe's elections not credible without an independent investigation into alleged vote-rigging.

"I am extremely concerned that the MDC had to withdraw its legal challenge due to concerns over the independence of the judiciary," British Foreign Secretary William Hague said. "I strongly believe that an independent investigation of any allegations of election irregularities would be required for the election result to be deemed credible."

He added that he had "grave concerns" over the conduct of the election and the "flaws" highlighted in the initial assessments by the South African Development Community and African Union observation missions.

"There is strong evidence that these elections fell short of SADC's own guidelines and the Zimbabwean electoral law," Hague said in a statement. "We are concerned about the potential implications for the region."

mkg/pfd (Reuters, AFP, dpa)