An autopsy report released today on the killing by police of 20 blacks near the town Uitenhage on March 21 revealed that 17 of them had been shot in the back. Within hours of the autopsy announcement, police headquarters in Pretoria said two more blacks were killed yesterday when a crowd of 800 attacked a police vehicle in the same area.

Police reported later that a group of blacks pulled a white man from his car in the white section of Uitenhage tonight and set him afire. The man, Erasmus Jacobs, 30, was hospitalized with severe burns.

According to the police announcement about yesterday's incident, efforts were made to disperse the crowd with tear gas, and when that failed, officers opened fire with rifles, leaving two persons dead.

The two deaths, and five reported later in eastern Cape Province townships, brought to 17 the number of blacks killed in clashes with the police since Saturday, as unrest has flared up anew in a dozen townships in the troubled region as well as some near Johannesburg.

The autopsy reports handed in today as evidence to the commission investigating the March 21 shootings showed that 17 of those killed were shot in the back. Ten of the 20 had brain wounds.

Nine were aged 16 and younger, including a girl of 11 and two boys aged 13 and 14. Only five were over 20.

Police have testified at the inquiry, which is being conducted by a provincial Supreme Court judge, Donald D. Kannemeyer, that as unrest in the eastern Cape townships worsened, they received orders March 15 to cease carrying tear gas and rubber bullets on their antiriot patrols.

According to the testimony, they were issued only semiautomatic R1 assault rifles and shotguns with heavy-caliber cartridges from that date.

The commission also has been told that two days before the March 21 Langa shooting, all police divisions were ordered to use the R1 rifles to "eliminate" any rioter seen throwing a gasoline or acid bomb.

The testimony has caused widespread concern in political circles here, with opposition parties calling for the dismissal of Law and Order Minister Louis le Grange, who is in charge of the police. President Pieter W. Botha has declared his support for the beleaguered minister and senior police officers have given assurances that riot squads are under instructions to use the minimum force needed to restore order.

Unrest in the eastern Cape Province townships, which had shown signs of dying down after the Langa shooting, flared up again last weekend when seven persons were killed in clashes with the police near the Indian Ocean city Port Elizabeth, neighboring Uitenhage and a number of small inland towns.

Another three were killed Tuesday. The seven shot to death yesterday brought the total killed in the region during the past three months to 82 and the national death toll since last September to nearly 300.

Police gave no immediate explanation today for the attack on Jacobs. A hospital spokesman said his condition was "serious but stable." A second man in the car was unhurt, police said.

According to the police statement, there was unrest in at least 12 eastern Cape townships yesterday, with crowds of blacks throwing stones and gasoline bombs at buses, school buildings and police vehicles.

Police reported later that a bomb exploded in a supermarket in downtown Durban in Natal Province. It caused damage but no injuries, police said.

Many of the demonstrators in the eastern Cape Province are students who are staying away from classes to protest what they regard as inferior education under the country's system of racial segregation.

Some clashes occurred when police moved in to disperse illegal gatherings, the police statement said.The gatherings are illegal because the government, in a bid to stop the continuing unrest, banned 29 black organizations from holding meetings three weeks ago. Most were organizations in the eastern Cape.