* President leads vote with wide margin

* Main opposition candidate says election rigged

* Commonwealth says playing field was not level

* Official results due Thursday evening

(Releads, updates results)

By Mabvuto Banda

LILONGWE, May 21 (Reuters) - Malawi's President Bingu wa Mutharika took a commanding lead in an election but opposition leader John Tembo accused the government of rigging the vote and said on Thursday he would challenge the result in court.

The president was well ahead with more than half the votes counted in an election viewed as a test for political stability in the poor but rapidly growing southern African country. Final results are due late on Thursday.

But Tembo cried foul and said he would go to court.

"I have received complaints from all over the country and believe that we have evidence to show that the election was rigged," Tembo, the main opposition presidential candidate, told a news conference.

Wa Mutharika based his campaign for a second term on his record of making Malawi a net food exporter and delivering three years of growth above 7 percent in the country of 13 million, where annual gross domestic product is only $313 per capita.

Electoral Commission head Anastasia Msosa said that with more than half the votes counted, wa Mutharika had 2.1 million votes while Tembo had about 733,000

"The incumbent president is still in the lead with a wide margin...the final results may be announced in the next two hours," Msosa said, adding wa Mutharika's Democratic Progressive Party won over half of parliamentary seats for constituencies counted.

The new president is due to be inaugurated on Friday.

A Commonwealth election monitoring mission earlier said wa Mutharika had exploited state media to gain an unfair advantage in the election but said the opposition should drop its protest.

"They should not do that, that is the nature of democracy, you are in today and out tomorrow," said John Kufuor, Ghana's former president and leader of the Commonwealth team.

MULUZI CONCEDES

Former president and leader of the opposition United Democratic Front (UDF) Bakili Muluzi, who was excluded from standing himself but had formed an alliance with Malawi Congress Party (MCP) Leader Tembo, acknowledged wa Mutharika had won and said he would support the new government.

Asked why he broke ranks with Muluzi, Tembo said each party had its own identity. "What UDF does and what MCP does, does not need to be the same in the electoral alliance," he said.

Muluzi has been an arch rival of wa Mutharika, and a protracted power struggle between the two almost paralysed parliament, prompting a failed impeachment bid and allegations of a coup plot that unnerved crucial Western donors.

Kufuor said in an interim report that while the election was in general well managed, state TV and radio was "unashamedly biased", meaning there was no level playing field.

The Southern African Development Community (SADC) also said there was media bias but said the vote was credible and free.

Malawi has the world's second-fastest growing economy, according to the Economist Intelligence Unit. But further political upheaval could hurt efforts to attract more foreign investors.

In parliamentary elections, which were also held on Tuesday, wa Mutharika won 74 of the parliament's 193 seats, with 118 constituencies counted. Tembo's MCP won 11 and Muluzi's UDF took 12, Msosa said.

(Writing by Rebecca Harrison; Editing by Jon Hemming)

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