By Humphrey Malalo

NAIROBI (Reuters) - Kenya's defeated presidential contender Raila Odinga filed a legal challenge to his election loss on Saturday in a major test of the country's democratic system five years after a disputed vote triggered deadly tribal clashes.

Police used teargas to break up a rally of about 100 of Odinga's supporters outside the Supreme Court earlier in the day, but he called on his allies to stay calm and trust in the law to resolve his complaint.

Odinga refuses to accept last week's slim first-round election win by Uhuru Kenyatta, Kenya's richest man and son of its first president.

Lawyers for Odinga's CORD coalition said their petition to the Supreme Court covered allegations of vote manipulation, problems in the registration of voters and the failure of an electronic vote counting mechanism.

"These failures dwarf anything Kenyans have ever witnessed in any previous election," Odinga told reporters on the doorstep of his office in central Nairobi.

He urged his supporters not to resort to violence. "We cannot begin what is supposed to be a new era under a new constitution in the same old ways," he said.

Kenya's March 4 election was the first since disputes over a 2007 presidential vote, which Odinga also lost, touched off tribal bloodletting in which more than 1,200 people were killed.

At least one person was wounded on Saturday morning as police fired teargas to clear Odinga supporters from the front of the Supreme Court building.

Some of the crowd wore T-shirts marked with slogans "I support the petition" and "Democracy on trial", an echo of Odinga's statement that his petition would be a test of democracy in the east African country.

Throughout Nairobi the traffic was moving freely and there were no signs of further unrest in the capital.

Kenyatta faces charges of crimes against humanity at the International Criminal Court alleging that he incited the 2007 violence. He denies the charges.

That violence sent the region's biggest economy into a tailspin and threatened the country's image as a safe place for tourists and investors.

(Writing by Drazen Jorgic; Editing by Andrew Heavens and Mark Heinrich)