* Supreme Court due to rule on Saturday
* Kenyatta, Odinga say will respect ruling
* U.S. Embassy warns citizens about possible unrest
Kenya's Supreme Court hears a petition on Wednesday challenging the victory by Uhuru Kenyatta in this month's presidential election, a case that will test Kenyan democracy five years after a disputed vote ignited tribal violence.
Peaceful voting on March 4 went a long way to restoring Kenya's reputation as one of Africa's more stable democracies, reinforced when losing candidate Raila Odinga took his challenge to court rather than letting it play out on the streets.
But the final test will come on Saturday, the deadline for the court to announce its ruling after petition hearings on Wednesday and Thursday. That is when the court will decide whether to uphold Kenyatta's win or order another vote.
Western diplomats, whose nations are big donors to Kenya, see little chance of another spasm of violence, partly because Kenyans now have more confidence that the reformed judiciary will adjudicate fairly. But the U.S. Embassy has still warned its citizens in Kenya about possible unrest this weekend.
Both candidates have promised to accept the outcome and many Kenyans insist there will be no repeat of the bloodshed that followed the 2007 vote which left more than 1,200 dead.
"We are alive to the competing expectations the Kenyan people have placed before this court," Chief Justice Willy Mutunga, a well-respected lawyer appointed to the post in 2011, said when he opened two days of pre-trial hearings on Monday.
Odinga alleged "rampant illegality" in the first-round vote. Kenyatta, son of Kenya's founding president who is charged by International Criminal Court with crimes against humanity over post-election violence five years ago, said voting was fair.
"The Supreme Court is open in order to enable the public to be witnesses to how the wheels of justice turn," Mutunga said, referring to television cameras covering the hearings with live broadcasts. "In the end, you must trust us to do our job."
Western donors have been watching the election closely, worried about the stability of a nation that has been an ally in the battle against militant Islamism in the region.
They have also said a Kenyatta victory would complicate relations because of the charges he faces in the Hague-based court. He denies the accusations and has promised to cooperate with the court to clear his name.
During Tuesday's pre-trial hearing, an attempt by Odinga to introduce fresh evidence showing malpractice in more constituencies was rejected on grounds that it should have been submitted earlier and would only delay the proceedings.
Odinga's plea for a forensic audit of the electoral commission's information technology systems, which failed during the elections, was also thrown out.
On Monday, the court ordered a partial recount of ballots from the vote after Odinga alleged there were more votes cast at some polling stations than there were registered voters.
Once it rules, the Supreme Court's verdict on the case is final.
Should Odinga win his challenge, it would mean a new presidential race that would further unnerve markets and prolong uncertainty. On the other hand, Kenyatta can only be sworn in if the court upholds his victory.
Kenya's police inspector general, David Kimaiyo, said on Tuesday that a ban on rallies and demonstrations was still in place as part of a bid to avoid violence. Rights groups have criticised the ban, saying it violates Kenyans' freedoms.