By Jeremy Clarke

NAIROBI (Reuters) - Kenyan Muslims marched on police headquarters in Nairobi on Thursday in protest against what they called the illegal detention and torture of fellow Muslims in an anti-terrorist drive urged on by the United States.

The protest involving a few dozen people followed months of simmering tensions between the east African nation's Muslim community and authorities they accuse of persecuting and arresting them on U.S. government orders.

"We don't expect this in our country. Just how much power do the Americans have over the Kenyan government?" said Al-Amin Kimathi, chairman of Kenya's Muslim Human Rights Forum.

Kenyan authorities could not be reached for comment.

The United States embassy in Nairobi said the United States was committed to working with regional partners to oppose terrorism.

"We are working to...disrupt the efforts of terrorists wherever they may be active," embassy spokeswoman Jennifer Barnes said.

Human Rights groups accuse Kenya of involvement in a clandestine U.S. practice of detainee transfer known as rendition.

AGGRESSIVE INTERROGATING

Kenyan police arrested scores of people on the Somali border in January and February after allied Ethiopian and Somali government troops chased Islamist fighters Washington accuses of having links to Al Qaeda out of Mogadishu.

Human rights groups say Kenyan authorities put dozens of terror suspects from Kenya on secret rendition flights to Ethiopia for interrogation by U.S. officials. Local activists said none had been prosecuted in any court.

"We know from a released prisoner that it is Americans doing the aggressive interrogating, and the Kenyan government is making it possible for them," Kimathi said.

At police headquarters, protesters demanded to know the whereabouts of two brothers who have gone missing.

They said Kenyan police took the younger brother to Somalia, then Ethiopia, in January without charge or explanation. He was able to contact relatives once to tell of his torture, the activists say.

Police seized his older brother last week outside a Nairobi mosque, according to relatives who were told nothing further and fear he faces the same fate as his younger brother.

Family members of the two brothers joined Thursday's protest and delivered a letter to police.

"The crack-down of so-called terrorists...is a blanket design and a veiled, skilful and state-orchestrated machination aimed at intimidating, harassing and persecuting members of the Muslim community," the letter said.