Nationalist guerrillas dug in along Rhodesia's mountainous eastern border have launched a campaign to get blacks to boycott the elections here next week for Rhodesia's first black-led government.

At guerrilla insistence, hundreds of blacks working on white farms south of here have left and gone back to their homes in the tribal trust lands or reserves, and black Africans crowded into the townships surrounding this garrison border town are coming under pressure to do the same.

Prime Minister Ian Smith's Rhodesian Front Party has already won all 28 seats reserved for whites in Parliament, assuring itself of a powerful voice in the future black-ruled country. Voting for the 72 black seats takes place separately.

Sunday night, the guerrillas balsted Dangamvura, a black suburb of Umtali, with mortars, rockets and small arms gunfire for half an hour, killing two blacks and wounding four others.

They then sent around teen-age messengers, calling upon the 40,000 residents of Dangamvura to return to their trust lands for the duration of the elections April 17-21.

But apart from this attack, there has so far been no sign of a major guerrilla offensive to disrupt the election, which nationalists contend will not really transfer power to the black majority.

"There is an ominous calm here," remarked the editor of the local newspaper, the Umtali Post.

Umtali, like the rest of the Rhodesia, has become edgy waiting for the expected guerrilla election offensive which still has not come.

"We do know that the terrorists have not yet exerted a major effort to frustrate the elections," Prime Minister Ian Smith said in a radio interview here Wednesday. But he said he anticipated a "crescendo" in the seven-year-old guerrilla war next week.

Officials here fear the guerillas may try to storm and hold one of the smaller white farming centers south of here like Cashel, located near the Mozambique border and now almost completely deserted.

The guerrillas, belonging to Robert Mugabe's Zimbabwe African National Union, have one of their main strongholds in this rugged terrain stradding the border with Mozambique. In fact, a number of the trust lands here are now largely under their control, and this explains why they are trying to get African voters back into them.

Unofficial white estimates of the black turnout go no higher than 40 percent of the 402,000 voters in eastern Mainicaland, one of the eight election districts into which Rhodesia has been divided.

The Rev. Ndabaningi Sithole, a member of the transitional government's executive council, and a former guerrilla leader himself, is running in his hometown area of Chipinga 80 miles south of here. But his influence is not expected to offset the guerrilla presence other than in his immediate hometown area.

One tactic the government is developing to bolster its weak position along the border is to pour army "auxilliaries" into the tribal trust lands with the largest guerrilla presence. At least 100 of these auxilliaries, most of them hastily trained and armed youth supporting the government, were sent to reinforce regular army troops in just one tribal trust land this week.

The government counteroffenseive in this area has been a long time in coming and may simply be too late to halt the progressive guerrilla takeover of the eastern border area. About half of the nearly 1,200 white farms in Manicaland have already been abandoned, according to a leading member of the farming community here, because of the guerrilla threat and a number of deaths among farmers.

Traffic now moves only in convoy to the south of here, and guerrilla ambushes are frequent all along the road to Chipinga and farther south to Fort Victoria.

The guerrillas have stopped all buses carrying black passengers from traveling along this road or into the tirbal trust and lands and often put up roadblocks to check traffic.

The seriousness of the guerrilla threat to the government in eastern Rhodesia was brought home to foreign correspondents here to cover the pre-election campaign by several minor incidents this past week.

In one, reporters trying to cover a rally of Bishop Abel Muzorewa in a tribal trust land near Rusape, 50 miles west of here, got no farther than the town itself. Muzorewa is widely expected to become Rhodesia's first black prime minister.

Police in Rusape said a big battle was under way between security forces and the guerrillas and that the dirt road leading into the reserve was too dangerous to travel.

In another incident, reporters invited to a Sithole rally in far northeast Mudzi tribal trust land bordering on Mozambique were stopped at Mtoko and told to go back.

even army auxilliaries loyal to Sithole and lounging about Mtoko refused to escort the group to the rally.

But the town of Umtali is calm to the point of being sleepy and seemingly half-deserted. The white population has decreased over the past two years from 10,000 to less than 7,000 while the number of blacks has jumped from 37,000 to more than 80,000, many of them war refugees.

In the windows of real estate agencies, there is a wide selection of homes and farms up for sale at bargain prices, including one three-bedroom villa with a "bomb shelter" and "natives' quarters" for about $13,000.

Umtali, military headquarters for the entire eastern border region, has been shelled by the guerrillas four times in the past three years from the surrounding mountains or across the border in Mozambique. Previously, it was the wite business and residential areas that were hit and about half a dozen whites have been injured.

[Reuter quoted informed sources in Lusaka as saying that Rhodesian planes attacked nationalist camps near the border town of Luangwa in the third raid into Zambia in 36 hours.] CAPTION: Picture, White Rhodesian Peter Rorbye addresses black farm workers as part of campaign to encourage voting. AP