Leaders of Raila Odinga's opposition Orange Democratic Movement (ODM) said they would hold a mass demonstration in a central Nairobi park next Wednesday.
"Due to the large number of people expected we request the police to provide us with security," ODM said in a statement.
Previous protests have led to riots and vicious clashes between Odinga's supporters and the security forces, adding to a total death toll of around 500 since the December 27 vote.
The government has said it will not allow more protests, and police in riot gear patrolled parts of the capital on Friday.
"Dialogue is not engaged in the streets. Dialogue suggests that people resolve their differences peacefully, over a table, not through destroying property and killing innocent Kenyans," Local Government Minister Uhuru Kenyatta told reporters.
The unrest has tarnished Kenya's democratic credentials, damaged east Africa's largest and previously booming economy, hit supplies to neighbours, and unnerved Western donors.
An opposition source said ODM leaders would march to a Nairobi police station later on Friday to hand over notice of their plans for nationwide demonstrations starting on Wednesday.
"We are going to go through a very dark period in this country's history," the source said.
This week's failure of African Union head and Ghanaian President John Kufuor to broker a deal has depressed Kenyans living through one of the worst chapters of their nation's post-independence history.
Kufuor flew out empty-handed late on Thursday, but said former U.N. head Kofi Annan, another Ghanaian, would lead a group of eminent Africans in another push to resolve the crisis.
