Prime Minister Robert Mugabe renewed his government's commitment to harmony between blacks and white today in a speech apparently aimed at counteracting white concern following the arrest of a key black aide charged with killing a white farmer.

Standing shoulder to shoulder with President Samora Machel of Mozambique, a staunch ally in Mugabe's long guerrilla war against white-minority rule, Mugabe told a soccer stadium rally that since his government came to power "blacks and whites can calm proudly that they belong to one united nation; in the past they could not."

Yesterday police arrested Edgar Tekere, 43, Mugabe's minister of manpower planning and a well-known former guerrilla leader, on charges he was involved in an attack Monday by a group of blacks on a farm outside Salisbury. The attack led to the shooting death of a white farmer, Gerald William Adams.

Although Mabe's speech today did not mention in incident directly, other goverment officials have pledged that justice will be done in the case. Former prime minister Garfield Todd, now a senator appointed by Mugabe, said Tekere's arrest would enhance the government's credibility.

Since coming to power three months ago, Mugabe has attempted to convince Zimbabwe's 200,000 whites to remain in the country. Tekere, however, was known to advocate a hard line against whites, and his arrest in Adams' death is likely to be seen by whites , concerned over a number of recent incidents of violence against whites, as a test case for the new government's pledge of nonracial rule of law.

A High Court Judge today upheld a lower court order refusing bail for Tekere. Six other unidentifield men also have been arrested in the case, and officials said up to 16 men would eventually be charged with the murder.

Jim Sinclair, a spokesman for the white Commercial Farmer's Union, said white farmers are concerned about continued sporadic violence in the countryside, the arena of most of the fighting during the seven-year civil war, but that there is no panic. He attributed recent attacks such as the murder of Adams to the "large number of people in the country who are armed and not gainfully employed at the moment."

Mozambique's President Machel, the first head of state to visit here since independence, will end his-five-day visit Friday.