The Kenyan opposition leader, Raila Odinga, has said he will hold a rally in the capital, Nairobi, despite a police ban.
Odinga supporters say he was robbed of victory
Security forces have surrounded the park where the rally is due to be held.
Refusing to accept defeat to Mwai Kibaki in the 27 December presidential election, Mr Odinga's party boycotted crisis talks with the new president.
More than 300 people have been killed and at least 70,000 driven from their homes across Kenya since Sunday.
Both sides have hardened their positions ahead of the planned mass rally on Thursday morning in Uhuru Park, the BBC's Grant Ferrett reports from Nairobi.
And hopes of outside mediation are fading with news that a visit by African Union leader John Kufuor to Nairobi is unlikely to happen, our correspondent adds.
Mr Odinga said the rally would send a peaceful message to supporters opposition.
Salim Lone, his campaign manager, told the BBC that his party was not courting trouble.
"The eyes of the world are on us for this rally, because everywhere Mr Odinga goes, he is asked are you going to allow this rally to happen," he said.
But Vice-President Moody Awori urged Mr Odinga to accept defeat and call off the protest.
"Please do not risk the lives of Kenyans, encouraging a large crowd of people coming in Nairobi on a working day," he said, speaking to reporters.
Riot police blocked opposition supporters trying to break out of slum areas to reach the city centre earlier in the week.
Supporters of Mr Odinga and President Kibaki have accused each other of genocide.
Mr Kibaki had invited all newly elected members of parliament to an urgent meeting at state house.
But instead of attending, Mr Odinga's Orange Democratic Movement MPs held a news conference to again urge Mr Kibaki to leave office.
MASS RALLY PLANS
Raila Odinga has called for a 1m-strong march in Nairobi
Marchers aim to rally in Uhuru (Freedom) Park, focus of pro-democracy protests in 1990s
The government is banning all public rallies in the immediate aftermath of the election
"How could we attend?" ODM secretary general Anyang Nyongo was quoted by AFP news agency as saying. "He is not a president but a usurper. It is genocide because police are killing people."
Mr Odinga himself said there could be no "dialogue with a thief", referring to the alleged vote-rigging which returned Mr Kibaki to office.
Speaking on behalf of the government, Lands Minister Kivutha Kibwana accused the ODM of planning, funding and rehearsing "genocide and ethnic cleansing" before the election.
The mutual accusation of genocide is a dangerous escalation of the rhetoric at a time of heightened tension, our correspondent notes.
Mr Kufuor, president of Ghana, had been expected to arrive on Thursday but a senior Kenyan government minister said on Wednesday the visit would not take place.
Finance Minister Amos Kimunya told the BBC the visit was not going ahead because there was no need for international mediation in a "Kenyan situation".
The comments are contrary to statements from Mr Kufuor's office that Mr Kibaki has invited him to Kenya.
Michael Ranneberger, the US ambassador, told the BBC World Service that Mr Kibaki and Mr Odinga had to work together to bring peace to Kenya, even if the election result was still being contested.
"This is a time when two of the greatest Kenyan patriots - the president and Raila Odinga - need to step forward and work out a practical way forward in the interests of the Kenyan people," he said.
Samuel Kivuitu, head of Kenya's election commission, told the BBC's Network Africa programme that he could not say for sure if Mr Kibaki had won fairly until he was shown the original records.
"I don't know until I see the records - the original records - which I can't see unless the court authorises it - if we can get authority from law allowing us to check whether these figures are correct, we'll do so," he said.
The full devastation and horror of this week's unrest emerged on Wednesday as journalists visited the charred slums of Nairobi and areas of western Kenya which saw tribal violence.
Fear in the night
The BBC's Karen Allen walked through the smouldering embers of the wooden church on the outskirts of Eldoret, in the Rift Valley, where some 30 people were burnt alive.
People have been trying to identify dead relations in a Kisumu morgue
The people sheltering there were members of President Kibaki's own community but other groups were also targeted.
Political rivalries have exposed ethnic tensions these past few days and communities that once lived side by side now torn apart, our correspondent says.
Patrick Nongyez of the local Red Cross said he had never seen anything like the church attack in Kenya.
As dusk fell, hundreds of people were preparing to sleep at the main police station for security.
Chemu Mungo, an Eldoret student, told BBC Radio 5 Live that people in the town felt the only safe place to go now was the bush.
MAIN ETHNIC GROUPS IN KENYAN PROVINCES