A black politician accused of large-scale atrocities in a tribal homeland north of here was killed when his automobile exploded last night, in what appears to be the second reprisal assassination by the black underground in just over a month.

In two other violent attacks, guerrillas raided a police station in another tribal homeland, or semi-independent territory, last night killing seven persons, including three black policemen, and a member of the administration of a third homeland was shot dead on his doorstep.

The attacks indicated that unrest is shifting to the rural homelands as the security forces intensify their crackdown on black activists in the cities under the state of emergency declared six weeks ago. Four homelands, set aside for members of black tribes, have been declared independent by the white rulers of South Africa, but no other nation has recognized their legitimacy.

The slain politician, Piet Ntuli, was interior minister and strong man in the administration of the small KwaNdebele homeland, which is scheduled to become nominally independent in December under South Africa's apartheid system of racial compartmentalization.

He had been accused of using a private army of vigilantes called the Mbokhoto to impose a reign of terror in the territory by beating up, abducting and sometimes murdering opponents of "independence," according to charges brought by relatives of victims. Black activists have struck back, turning the situation into a virtual civil war and one of the most gruesome sideshows in South Africa's political conflict.

According to the Bureau for Information, Ntuli was killed when the automobile in which he was traveling exploded in Siyabuswa, KwaNdebele's capital. He was alone in the car.

A bureau spokesman said it was not yet known who was responsible for the attack, but local observers said it was almost certainly guerrillas of the underground African National Congress.

Five weeks ago gunmen of the congress carried out a similar killing when they shot dead the black police officer who gave the order to fire on protesters on a football field last March in another tribal homeland called Bophuthatswana. Eleven persons were killed.

The killing of the police officer, Brig. Andrew Molope, in the Winterveld squatter camp where the massacre took place, was the first reprisal assassination of a public official in South Africa.

Local sources said at least 160 people have died in KwaNdebele, which is about 70 miles northeast of Pretoria, since an uprising began there May 12 in opposition to the tribal administration of Chief Minister Simon Skhosana and its plans to accept Pretoria's offer of independence.

Most of the deaths have been in clashes between the heavily armed Mbokhoto vigilantes led by Ntuli and militant young activists known as "comrades."

Administration in the territory has broken down since civil servants began a strike two weeks ago demanding the resignation of Skhosana's Cabinet and an end to the independence plans. All the homeland's schools are closed.

Pretoria has imposed the most severe restrictions of the emergency on KwaNdebele, setting up roadblocks and declaring the 280-square-mile territory off limits to all nonresidents.

Curfew restrictions amounting to partial house arrest for the entire population of 500,000 have been imposed from 9 p.m. to 5 a.m. Goods may not be delivered to the territory without the permission of the commissioner of police, and children are not allowed to leave the districts where their parents live. No person may "play, loiter or aimlessly remain on any public road."

Behind this iron curtain, local sources say, Ntuli's Mbokhoto has been committing atrocities. There have been reports of hundreds of schoolchildren being abducted to detention camps where they were forced to walk on burning coals, and of the vigilantes enforcing circumcision on adult men who have not been through tribal initiation ceremonies.

Charges of assault and murder were laid against Ntuli after he led a vigilante raid on a neighboring territory called Moutse, which is resisting incorporation into KwaNdebele. In that raid, 26 persons were killed and hundreds abducted.

The interior minister had twice before stood trial for murder, once for killing his night watchman and once for killing a rival politician. Each time he was acquitted on a technicality.

In the case of the killing of the rival politician, Andries Mahlangu, Ntuli's son, Samuel, testified against him after requesting police protection because he said his father had threatened to kill him.

At the time of his death, Ntuli faced charges of large-scale theft of automobiles and of having an arms cache at his home. Although he was interior minister, he had to report to the police at Siyabuswa three times a week.

The attack on the police station was in Umtata, capital of the nominally independent Transkei tribal homeland in eastern Cape Province. A spokesman there said guerrillas armed with grenades and Soviet AK47 automatic rifles attacked the police station late last night, killing four civilians and three policemen.

Three gunmen knocked on the door of Zebulon Kunene, a member of the Kangwane tribal homeland administration in eastern Transvaal province, last night and shot him dead when he opened it. Police in the region said they had not yet established a motive for the killing.