Massive boulders crashed onto houses Wednesday in Mathare, a Nairobi slum, killing at least eight people after a night of heavy rains, police and the Kenyan Red Cross said.

"Eight people have died in this disaster. Six bodies were retrieved from under the rubble, another died on the way to hospital while the eighth died in hospital," a police officer involved in the rescue operation told AFP.

Hopes of retrieving any more people alive from the rubble faded late in the afternoon after bulldozers cleared the site.

"We do not believe that there are any more bodies here," the head of the Kenyan disaster management service Vincent Anami said.

The Kenya Red Cross said rescue teams' efforts had been hampered by heavy rains, lack of appropriate machinery and crowds of people milling around.

The Red Cross said more than 40 houses were destroyed in the sprawling slums in the north of the capital when huge boulders came crashing onto them early Wednesday morning.

Teams of rescue workers used hoes and axes in an attempt to break up boulders that towered over their heads as crowds of onlookers stood perilously close. Earth moving equipment was later brought in.

"The rescue efforts have been hampered by heavy rains, lack of access and appropriate machinery and crowds of people who are slowing down the whole process," the Red Cross said.

Teams from the military joined in the rescue effort in early afternoon.

Residents wailed and shouted as bulldozers roared at the scene lifting heavy rocks to enable rescuers check for bodies underground.

A witness who survived the early morning disaster told journalists how massive rocks crumbled and flattened shanty houses right next to his door step.

"I was asleep when I heard a loud crash and thought it was a thunderstorm. I immediately heard screams and people shouting, there was confusion all over and when I went outside it was each man for himself," said Kennedy Thuku who has eked out a living selling boiled eggs in Mathare for the past 17 years.

"I thank God for being alive today because most of those dead are people I know," he said.

"It's by God's grace that we are alive with my family," said Rose Okoth, a mother of three who also managed to run out of her house in time.

"I heard a loud thud. I thought it was a thunderstorm then I heard screams and that is when I ran out to safety with my children. A few minutes later another rock fell on our house. It could have killed us," she told AFP.

Heavy rains have begun pounding several Kenyan regions at the start of the main rainy season, which often causes floods and massive displacement.