A Kenyan human rights group said at least 24 people have been killed by police gunfire since the election on August 8. Violence erupted after opposition leader Raila Odinga said the election was rigged.

Mr Odinga said there was "a plot to kill our supporters" and that opposition backers should protest by not going to work on Monday. He also promised a major announcement on Tuesday.

He spoke to a crowd in Kibera, a Nairobi slum where his supporters battled police who fired live ammunition and tear gas over the past few days.

Kenya's election commission, which declared President Uhuru Kenyatta had won, says its voting process was fair.

Opposition strongholds hit by deadly violence over the election were quiet on Sunday, with many people attending church services and police patrolling some streets.

Pastors delivered sermons appealing for calm in the Nairobi slum of Mathare, where rioters have battled police who fired live ammunition and tear gas.

The pastors asked congregations to help rebuild and leave matters to God even if they feel they have been victims of injustice.

Outside the churches, which are made of little more than wood frames and tin roofs, children played soccer, darts, checkers and other games.

Similar scenes unfolded in the capital's Kibera slum, another site of recent clashes. The city of Kisumu, an opposition stronghold, was also calm.

The office of Mr Kenyatta said the recent protests were violent and unlawful.

Presidential spokesman Manoah Esipisu said any peaceful protests are a constitutional right and would be protected by police.

"But sadly, we have seen violent protests, in which property has been damaged, and lives have been endangered," he said.

"The police will not tolerate breaches of the peace; instead, they will protect the lives and property of Kenyans and they will restore law and order."

The Kenyan police force is denying the report that its officers killed 24 people in the election violence.

The National Police Service said on Facebook Sunday that police killed six "criminals" who were looting and rioting and attacked police officers in the past two days.

Police said they are aware of the shooting death of a nine-year-old girl in a Nairobi slum and are investigating, but added the force is being targeted by false allegations designed to raise tensions.