"We are resuming our peaceful public rallies on Thursday," Henry Kosgey, the chairman of the Orange Democratic Movement (ODM), told reporters.
Three days of opposition protests that began on Wednesday, despite a government ban, provoked a fierce crackdown by anti-riot and paramilitary police.
Al Jazeera's Bernard Smith in Nairobi said the violence in the Rift Valley was symptomatic of the political tensions that are simmering across the country.
"We understand that some vigilantes that were taking part in the protests last week were arrested by police and some others went to try and get them released," he said.
"Our sources tell us that the police resisted this, which is what prompted this group to go on the rampage."
A local reporter told the Associated Press news agency that he saw 17 people who had died from machete and arrow wounds and five shot by the police.
The police appeared to have quelled the violence by Sunday afternoon.
In the Mathare slum of Nairobi, homes were set on fire during several hours of running battles between rival Kikuyu and Luo tribes, a resident said.
Raila Odinga, the ODM leader and the opposition figure who Kibaki defeated in the disputed vote, is a Luo, while Kibaki is a Kikuyu.
Moses Ogolla, another resident, said that he saw four bodies with deep machete cuts being put into a police vehicle.
"I think it was a gang who attacked them because some bodies, the head had six, seven, eight cuts on it," Ogolla said.
Call for peace
Meanwhile, Odinga has called for an end to the post-election violence that has killed an estimated 650 people.
"No need to kill somebody because of his tribe, even if he did not vote for me," he said as he left a church service in Nairobi's Kibera slum on Sunday.
|About 650 people have died in |
post-election violence [AFP]
Odinga said a memorial service would be held at a sports field in central Nairobi on Wednesday for those who had died and repeated the call for more demonstrations from Thursday, despite police orders to prevent rallies.
"You can beat our body, but you cannot break our spirit of justice," he told supporters, some holding up banners reading "Raila our solution" or "Kibaki hand over to Raila".
Both the ODM and the government have accused each other of inciting genocide.
The two rivals have agreed in principle to hold talks, but Kibaki has said he wants direct talks with Odinga, while the opposition leader says he will negotiate only through a mediator who can provide an internationally guaranteed agreement.
Kofi Annan, the former UN secretary-general, is due to arrive in Kenya this week to bring the two parties together for talks.
Louis Michel, the European Union's aid commissioner, who met with Kibaki and Odinga, urged both sides to start talks to end their standoff.
"Mass meetings ... can lead to aggression which can also lead to powerful responses," he said.