By Wangui Kanina and C. Bryson Hull

NAIROBI, June 26 (Reuters) - Kenyan police on Tuesday said they had shot dead at least 25 suspected members of the Mungiki criminal gang since last week, after at least 13 people were killed in a surge of violence blamed on the group.

Thursday's conviction of a former Mungiki leader on weapons charges ended a brief lull in the slaughter.

Fatalities and suspects have piled up quickly since then, in incidents ranging from a grenade attack on a Nairobi bar that killed five, to beheadings with machetes, the discovery of severed heads and genitals, and bloody police retaliation.

"It is a pity that our contacts have to be so bloody, but you know these people have sworn an oath that they will never surrender," police spokesman Eric Kiraithe said.

With President Mwai Kibaki's blessing, Kenya's police have promised to wipe out Mungiki following a series of killings since late March in Nairobi and central Kenya.

Many fear the group plans mayhem during this year's presidential election, expected in late December.

In recent days the police crackdown has intensified.

Police said they shot dead two suspected members as they prepared to rob a bank in the capital Nairobi on Tuesday and another five had been killed over the weekend.

Local media said officers in Thika, about 60 km (37 miles) north of the capital, on Monday killed two others and recovered military gear, assault rifles and Ceska pistols -- a standard police sidearm in Kenya -- at a hideout there.

Kiraithe could not immediately confirm the Thika killings and seizures.

MAU MAU RITUALS

Since declaring war on Mungiki, police have intensified manhunts and carried out a bloody siege on a gang stronghold in early June that killed at least 33 people.

The group rose to prominence in the early 1990s espousing a return to the traditional values of the Kikuyu tribe, Kenya's largest and the backbone of the Mau Mau rebels that fought the British colonial government for independence.

They adopted Mau Mau rituals like taking oaths. But police and experts say they have now evolved into Kenya's biggest criminal mafia with tentacles reaching into politics, where ethnic-based violence has long been an electoral tool.

Human rights groups have accused police of indiscriminate slaughter and of branding any potential criminal a Mungiki member to justify harsh action.

Government officials admit privately Mungiki has connections in the upper spheres of politics and the security apparatus.

Either way, extortion from Kenya's lucrative minibus transport industry and other protection rackets have given it enough money to buy protection in a nation where corruption is a daily fact of life for most people.

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