Ten months in a Nairobi jail . . .

Why is Kenyan lawyer John Khaminwa behind bars? That question is a focal point of concerns about human rights in Kenya and what appears to be a narrowing of the government's tolerance of dissent over the past year.

Mr. Khaminwa is one of about 12 people imprisoned under the president's powers to detain without trial. He was arrested last June but has not been charged with any crime.

Compared with many other African states, Kenya's practice of detaining opponents is markedly restrained, observers in Nairobi say. But Western diplomatic sources concede privately that Khaminwa's imprisonment has put a ''blot'' on the country's otherwise ''pretty reasonable'' human rights record.

Khaminwa, educated in Tanzania, Britain, and the United States, earned respect in the international legal community for his willingness to take cases regardless of their political sensitivity. That practice also gained him a number of enemies in the Kenyan government.

Prior to his own detention, Khaminwa was representing two controversial detainees: a former deputy director of intelligence and a former member of Parliament. Perhaps Khaminwa's best-known past client is Oginga Odinga, Kenya's first vice-president but later a consistent government opponent.

The final act that may have gotten Khaminwa in trouble, some say, was his challenging in court of the president's power to detain without trial.

The government has given no reason for Khaminwa's detention, but criticism has mounted that Khaminwa was imprisoned simply for ''doing his job'' as a lawyer. This is unlike the imprisonment of many detainees arrested for political views, observers say.

There is no official press censorship in Kenya, but the government has clearly intimidated many journalists. ''Lots of them are imposing self-censorship,'' a Nairobi journalist says. In a separate action, the ruling Kenya African National Union has taken over the Nairobi Times, planning to use it as a party mouthpiece.

In contrast to its past tradition of permitting a wide range of dissent, Kenya seems to have established clear limits on how much criticism will be tolerated.