By Jeremy Clarke

NAIROBI, June 10 (Reuters) - I arrived on the hills above Nairobi's Mathare slum expecting to see a violent anti-gang operation winding down but saw instead hundreds of heavily-armed Kenyan police beating the cowering residents.

The crack of gunfire rang out, thick teargas filled the air, and scores of women and children cried and ran to escape the whips and batons of the police.

I stumbled down the trash-littered slopes where pigs bathed and goats picked for food in a place that is home to 300,000 people and a stronghold of the notorious Mungiki gang.

It was the fourth day of a retaliatory operation for the killing of two police officers. Hundreds of men with assault rifles besieged Mathare, where two days earlier they shot dead 22 Mungiki suspects they said were resisting arrest.

The nation's biggest criminal organisation has been front-page news for weeks with a series of beheadings, mutilations and murders of policemen and government officials.

Mungiki began as a quasi-religious sect in the 1990s but is now Kenya's biggest, most ruthless mafia.

It has threatened to overthrow the government, raising alarm ahead of this year's presidential vote in a country where elections rarely pass without violence.

President Mwai Kibaki and his tough-talking internal security minister, John Michuki, promised to wipe the gang out.

NOT UNDERCOVER

The police had told me not to bother coming back to Nairobi's Mathare slum after three days.

"The operation will be much more undercover from now," spokesman Eric Kiraithe told me as the sun set over the valley of tin huts and sewage. "We are not violent like the criminals. We don't want to disrupt the lives of innocent people."

Despite Kiraithe's assurances I returned. I heard gunfire and when a bullet seemed to land behind me, I dived to the mud.

I picked myself up to jeers, realising the sound was slum-dwellers tearing down their huts at gunpoint.

I turned the corner and saw groups of people piled face-down in the bloodstained dirt as police stood over them, whipping and caning. The ground was flecked with bits of flesh, apparently from a shooting victim.

I saw a group of police force a boy of about 10 to drop his trousers for a beating. Other officers hit a young girl, in full Islamic dress, with rubber whips.

They tossed teargas canisters through open doors, sending choking occupants rushing out.

Other residents were put to work in the stinking river, trawling for body parts and police guns that were taken after the two officers were ambushed.

One woman carrying a baby on her hip, slow to do what she was told, was hit in the throat by a club-wielding officer. She fell where others were bleeding from gaping head wounds.

I found myself on the wrong end of a police kicking when I offered a comforting hand to people on the ground, my heavy Australian accent no good for shouting down a gun barrel at a paramilitary officer.

By mid-afternoon, I had seen locals forced to ferry at least 11 of their dead neighbours in makeshift sacks trailing blood, and heap them in police trucks parked above the valley.

Blood dripped like oil from one pick-up as body after body was dumped in, bringing the toll from police action to 33.

Pools of blood greeted hundreds of children returning from school to find cowering parents.

Police say they are using justified force, and many Kenyans weary of violent crime agree with an approach that is summed up in the Kiswahili proverb: Dawa ya moto ni moto -- the cure for fire is fire.

As the police withdrew at day's end, one officer told me: "You journalists must like this, eh? Mission accomplished, we have successfully recovered our two guns."

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