A diplomat for Rhodesia when it was still a colony of Britain was today elected president, a ceremonial post, under Rhodesia's limited black majority rule government.

The selection of Josiah Zion Gumede, 59, a member of the minority Ndebele tribe, is seen as an attempt by Bishop Abel Muzorewa, the prime minister-elect, to extend his following among the Ndebeles.

Gumede was nominated by Muzorewa's United African National Party, which draws its support largely from the Shona tribe that comprises about 80 percent of Rhodesia's 6 million blacks. Historically Shonas and Ndebeles have been rivals.

In addition, one of the guerrilla leaders who opposes the black-led government elected last month, Joshua Nkomo, has widespread support among the Ndebele. Many of the tribe boycotted the April election.

Yet Muzorewa's government, scheduled to take power Friday, still faces a serious problem with the refusal of two minority parties to participate in his Cabinet.

Rev. Ndabaningi Sithole, a signer of the March 1978 agreement by which Rhodesia's white rulers agreed to turn over power to blacks, has refused to join Muzorewa's government. He claims the elections, in which his party won 12 seats in the National Assembly compared to Muzorewa's 51, was marred by "gross irregularities."

His party has refused to take its 12 seats and to accept the two Cabinet posts it was to receive.

Tonight another minority party led by Chief Kayisa Ndiweni, announced it also was withdrawing its support for the Muzorewa-led government and was declining the two Cabinet posts it was to receive. Ndiweni's party will, however, occupy its nine seats as an opposition. Ndiweni had objected to Muzorewa's nomination of Gumede.

Gumede's election today by the new legislature is the first step in a weeklong constitutional transfer of power. The president is expected to be sworn in Tuesday and to immediately name Muzorewa prime minister.

Wednesday Muzorewa is scheduled to name his Cabinet, which must contain at least six whites from outgoing Prime Minister Ian Smith's party that holds all of the 28 assembly seats reserved for Rhodesia's white population of about 230,000.

Muzorewa and his Cabinet will be inaugurated formally on Friday. That same day the new constitution providing for black rule with substantial political guarantees for whites will take effect. umede served as assistant information and education attache in the Rhodesian government offices in London when the country was still part of the British-ruled Central African Federation, which then was made up of Rhodesia, Malawi and Zambia.

He was later promoted to first secretary at the Rhodesian government offices in Nairobi. He resigned from the diplomatic service in 1965 in protest of Smith's declaration of independence from Britain and joined the British civil service until he returned to Rhodesia.