Elections for Rhodesia's first black-dominated Parliament start here Tuesday with all adult African voting for the first time in the 90-year history of this breakawy British colony to finally bring an end to all-white rule.

In the midst of a bloody guerilla war, the Rhodesian government has mobilized 60,000 police and Army troops to Protect the potential 2.8 million mostly black voters, 70 interntional observers and 250 foreign journalists here to witness the historic five-day event.

Confident now of a better than 50 percent turnout, some black and white Rhodesians seems to be riding high and further buoyed by the prospect of a new Conservative government in Britain and an apparently swelling tide of support for their cause in the U.S. Congress.

The Guerrillas, who have denounced the election as a sham that continues white minority control over the country, launched a rocket attack against Rhodesia's second-largest oil refinery last night.Refinery officials said some of the tanks caught fire but refused to say how much fuel was lost.

The fires reportedly were brought under control seven hours after the attack. A few of the firefighters were said to have suffered minor injuries.

Elsewhere guerillas attacked five polling places.

Guerillas shellded a Salisbury bus depot Monday night, damaging several buses likely to have been used to ferry black voters to the polls, United Press International reported.

[A Police spokesman said no one was injured.]

The election is to choose the 72 black members who will sit in the 100-seat Parliament alongside 28 whites. Twenty of the white members, led by Prime Minister Ian Smith, have already been elected separately and the other eight will not be chosen until a special slectoral college meets here May 7.

In election eve statements, the two leading contenders to become Rhodesia's first black prime minister, Bishop Abel Muzorewa and the Rev. Ndabaningi Sithole, appealed to Britain and the United States in particular to recognize the new black-led government.

"I challenge the Americans, the British and their fellow travelers," said the bishop, "to admit the objections they may have had against this country in the past have no further validity. They have only one true and honest course, to immediately recognize our black majority government, democratically elected."

Sithole said at a press conference that the elections will no doubt usher in a very practical manner a new social order which is replacing an old one based on racial discrimination and white minority rule. We shall appeal to the outside world that they should change their attitude toward us because the new social order is based on nonracialism and black majority rule."

Observer groups coming from the United States include the Heritage Foundation, the Georgetown Center for Strategic Studies, the American Conservative Union, the Americans Security Council and Freedom House. Congress voted down an attempt to send an official U.S. delegation.

Included in the Freedom House delegation are five political scientists, among them Maurice Woodard of Howard University and Howard Penniman of Georgetown University. In addition, there is black civil rights leader Bayard Rustin and former U.S. delegate to the U.N. Human Rights Commission, Allard Lowenstein.

Rustin had some harsh words for Rep. Stephen Solarz (D.-N.Y.), chairman of the House Subcommittee on Africa, who finished up a two-day visit here today after leading the movement in Congress that blocked any official U.S. observers.

Calling him "the little boy from Brooklyn," Rustin said, "He's got his problems" and "I hope he has learned something."

The elections are part of a complicated two-month-long process by which Rhodesia's 6.8 million blacks will take over power from 230,000 whites under a constitution that still ensures for Europeans extensive political, military and economic influence, carefully protects their rights and gives them disproportionate representation in Parliament for at least 10 years.

Guerrilla leaders robert Mugabe and joshua Nkomo are dead set against the new constitution because they say it has entrenched continued white power and was never approved by Rhodesia's African population.

Bishop Muzorewa defends the constitution, saying it has given blacks what they have always wanted, majority rule and power. "It is power. That is the most important thing that we want, just get that power."

The constitution has not been an issue in the campaign, however. The only issues were whether blacks will vote and if so, for which of the five parties taking part in the race.

These are Bishop Muzorewa's United African National Council; Sithole's Zimbabwe African National Union; the Zimbabwe United People's Organization, led by a traditional chief, Jeremiah Chirau, and two smaller parties.

The Muzorewa and Sithole groups are expected to get practically all of the seats.

A key question for the new government and the international community is what percentage of blacks participate in the voting.

Estimates vary anywhere between 30 and 70 percent, with the lowest turnout expected in the most guerilla-ridden border districts of eastern Manicaland and western Matebeleland.

Sithole predicted that 70 to 80 percent of Africans living in urban areas would vote and 55 to 65 percent of those in the rural areas. This would give a national average of around 67 percent.

Most outside preelection estimates are between 50 and 60 percent, based on the number of Africans holding regular jobs or living in urban areas where it is easiest to get voters out.

The government has mobilized all farmers, home owners, industrial workers, company executives, civil servants and teachers to help get blacks to the polls.

At the same time, it has effectively silenced anyone opposed to the elections.

Not a word of dissent has been heard inside the country, although university students who had a press conference canceled last weekend are planning a demonstration on campus Tuesday against the elections. CAPTION: Picture, Buses are halted at a roadblock outside Salisbury to check for weapons being sent to terrorists in the city.