At least two people were killed and 61 others injured after police opened fire to disperse protesters demonstrating against Kenya's election body in the western Kisumu city, witnesses and an opposition official said.
Police fired into the air to break up the protest which was held near the Kisumu office of the Independent Electoral and Boundaries Commission (IEBC) on Monday, witnesses say.
Protesters also gathered on the streets of capital Nairobi.
The opposition CORD alliance has been organising protests every Monday since last April.
The CORD accuses the electoral commission of being pro-government and says the commission cannot guarantee democratic elections.
They demand resignation of its members before the presidential election.
However, on Sunday, Nairobi's police chief Japheth Koome warned that police were prepared to use lethal force if it was necessary.
"No demonstration. That's the message. If you have nothing else to do, sleep. It will not be allowed," Koome told the BBC.
"That's why I'm questioning those who have intentions of demonstrating... 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," he said.
Last month police officers killed three people in similar protests. The police said they were acting in self-defence.
Kenya will hold a presidential election in August 2017.
It is expected to spark a battle between current President Uhuru Kenyatta and opposition leader Raila Odinga.
CORD leader Raila Odinga blamed the commission for his defeat against Kenyatta in 2013.
Ahead of the presidential election next year, some concerns have been raised due to possible political or tribal violence.
Tribal violence after the 2007 presidential election left more than a thousand dead in the country.