( CNN ) - Kenya's opposition leader Raila Ondinga vowed to go ahead with a banned "million man" protest rally in Nairobi Thursday, setting the stage for further clashes between security forces and Odinga's supporters, the Associated Press reported.
The political and ethnic violence sweeping Kenya has already left as many as 300 people dead and displaced 100,000, according to humanitarian groups.
Odinga accuses President Mwai Kibaki of stealing the East African country's highly contested December 27 vote, and his Nairobi rally is expected to bring hundreds of thousands of supporters and their rivals onto the streets of the capital.
The government has banned the march.
Odinga told The Associated Press on Wednesday that his peaceful rally was meant "to communicate to our people, to inform them where we are coming from, where we are and where we want to go."
Meanwhile, post-election violence in Kenya brought international pleas for calm Wednesday amid mounting concerns over the legitimacy of President Mwai Kibaki's narrow victory in last week's vote.
Kenyan government officials said at least 209 people had so far been killed and around 75,000 forced to flee their homes as gangs of machete-wielding young men roamed the streets.
The Associated Press said more than 300 people were dead amid reports of horrific attacks, including the torching of a church where people who had sought refuge were burned alive.
Tensions could worsen Thursday when opposition leader Raila Odinga plans a huge protest rally in Nairobi.
"All Kenyans are invited to Uhuru Park," a statement on Odinga's campaign Web site said, and some party officials said at least a million people would attend.
The government has banned all rallies and Kenya's police commissioner sent a reminder Wednesday, telling citizens the gathering "has not been authorized and is therefore illegal."
Meanwhile, in an article published Wednesday in The Herald, Kenya's oldest newspaper, the head of the country's electoral commission, Samuel Kivuitu, was quoted as saying he did not know who had won the election.
Kivuitu said he had been pressured to announce the results, AP reported. His remarks came after international election observers voiced doubts over the conduct of last week's vote.
Ghanaian President John Kufuor, the head of the African Union, was due to arrive in Nairobi, the Kenyan capital, on Wednesday to act as a mediator, an AU spokeswoman told AP.
U.S. State Department spokesman Sean McCormack said Wednesday that Secretary of State Condoleezza Rice had spoken to Odinga and would speak with Kibaki later Wednesday.
"It is important for the Kenyan people and their democracy to work within the confines of their law and constitution to find a political way forward and, most importantly, to bring about an end to the violence," McCormack said.
"It's hard-pressed to comprehend here how this could have gone so wrong in terms of Kenya being on its way to some stability and then having this election turn into such a violent situation," said White House press secretary Dana Perino in a statement on behalf of the Bush administration.
Much of the violence is between supporters of Kibaki, from the majority Kikuyu tribe, and backers of Odinga, who is from the Luo tribe.
The ethnic violence, previously rare in Kenya, is reminiscent of the strife that led to the genocide in Rwanda in 1994.
In a particularly disturbing incident, a mob appears to have burned a church filled with Kenyans seeking refuge from the violence in the city of Eldoret in the country's western Rift Valley region.
The Red Cross told AP that at least 50 were burned to death at the church, some of them children.
As many as 200 people were at the church, about 185 miles ( 300 km) northwest of Nairobi, KTN reporter Tony Biwott told CNN. Video Watch as machete-wielding looters haul away goods "
Biwott said he had counted at least 15 charred bodies, including children, in the burned church and an adjacent field. "I'm sure there were more than 15 but I couldn't count the ones who were ashes," he said in a phone interview.
The wounded suffered gunshot wounds, burns and cuts from a panga, a machete-like weapon, the Red Cross said. Video Watch smoke darken skies near a burned church "
Kenya's national police commissioner has said an investigation into the incident is under way and expressed shock that the violence had occurred in a church.
Jayne Samuels, an American volunteer worker based in a village near the western city of Kisumu, told CNN she had been stranded in a hotel for two days as police clashed with rioters on the streets before finally catching a flight out of the country via Nairobi.
"I don't know what will happen, it's still volatile there right now," Samuels said. Video Watch how people are fleeing the violence "
Police and political backers of Odinga began clashing about four days ago as Odinga narrowly lost Kenya's presidential election to Kibaki.
Violence broke out in several cities as frustration mounted during the slow hand-count of the ballots.
Kibaki was re-elected with 51.3 percent of the vote, to 48.7 percent for Odinga.
"What we now witness is a cold and calculated plan to organize and engage in massacres," government spokesman Alfred Mutua said.
Bringing in the New Year, Kibaki -- who rarely speaks to the press -- urged calm to the nation.
"It now is a time for healing and reconciliation amongst all Kenyans," he said.
On Tuesday, international observers said the balloting fell short of international standards for democratic elections.
Alexander Lambsdorff, the head of the EU Election Observation Mission in Kenya, cited discrepancies in vote counts, election observers being turned away from polling places and observers being refused entrance to the electoral commission vote-counting room.
The government said Tuesday it would not allow any political rallies in the aftermath of the controversial election outcome. Foreign Minister Raphael Tuju said the government was committed to taking control.
"If the tear gas doesn't work then unfortunately they have to use live bullets," he said. "The president has been sworn in, the elections are over, the Kenyans have to accept the results, the opposition has to accept the results."
U.S. State Department spokesman Sean McCormack said Wednesday that Secretary of State Condoleezza Rice is urging Kenyan leaders to end political tensions in the aftermath of the recent election and stop the violence.
He said she already had talked to opposition leader Rail Odinga and was planning to speak to President Mwai Kibaki later Wednesday.
"She's going to urge both gentlemen to do everything they possibly can in the name of political reconciliation in Kenya to bring an end to any political tensions that might give rise to violence in that country," McCormack said at his afternoon briefing.
"We still have some concerns about irregularities involved in the vote counting. That said, it's important for Kenya to move beyond this," he said. "It is important for the Kenyan people and their democracy to work within the confines of their law and constitution to find a political way forward and, most importantly, to bring about an end to the violence."