Nairobi's police chief has warned opposition supporters not to take part in Monday's protest in the Kenyan capital "if you value your life".
Japheth Koome said police were prepared to use lethal force if necessary.
Opposition politician James Orengo said the rally would go ahead as there was no legal justification for banning it.
The opposition has been protesting every Monday for several weeks about the electoral commission, which it says is biased.
"No demonstration. That's the message. If you have nothing else to do, sleep. It will not be allowed," Mr Koome told the BBC.
"That's why I'm questioning those who have intentions of demonstrating tomorrow. Don't come, then you start talking about police brutality. Life is dear! Please, if you value your life, don't attempt it that way. You will be dealt with firmly."
At least three people died in western Kenya during similar protests.
Police said two people were shot dead in self defence.
A third person died from a head injury after falling over while running from tear gas.
Last month, there were complaints of police brutality as officers were pictured assaulting demonstrators in Nairobi.
The opposition Cord alliance wants the current members of the electoral commission to resign ahead of next year's presidential election.
Cord is complaining that they are biased in favour of the governing Jubilee coalition and wants it replaced by a neutral body.
Deputy President William Ruto told the BBC's Focus on Africa TV last month that Monday's protests were illegal, on the basis that police were not given adequate warning.