An uneasy calm descended tonight on most of the besieged black and mixed-race townships east of Cape Town after a third day of sporadic unrest that brought the death toll since Wednesday to 28 amid new charges of police brutality.

Meanwhile, the arrival of three European Community foreign ministers for a fact-finding visit and a decision by South Africa's official opposition party to participate in elections for the country's segregated mixed-race and Indian parliaments highlighted the deep political divisions among opponents of white-minority rule here.

Most of the Cape Town violence today centered on the mixed-race or Colored townships of Mitchell's Plain, Manenberg and Bellville where a number of buildings including post offices, a library, public schools and welfare offices were attacked during the night and where police and youths battled throughout the day.

Police used shotguns, rubber bullets and tear gas to disperse crowds that heaved rocks, bottles and occasional gasoline bombs at armored police cars and other vehicles.

In Mitchell's Plain youths wearing ski masks and handkerchiefs to conceal their faces set fire to several trucks and erected new barricades of burning tires, mattresses and refuse around the township. Several barricades were formed from bus shelters and broken desks from closed public schools that yesterday were the scene of running battles between police and students.

Teachers at a Bellville secondary school said police invaded school grounds today firing tear gas and arresting two pupils. The principal and teachers issued a statement condemning "today's display of brutality and indiscriminate action by the police" and noting that before today, "the situation at the school was peaceful and controlled."

The families of several children and young adults shot dead by police during the unrest also accused the authorities of using excessive force, as did staff members and riot victims at Empilisweni Sacla Clinic where more than 90 injured persons have been treated. These allegations were added to charges that police attacked children not involved in the violence yesterday in assaults on school grounds throughout the area.

Police tonight responded to the allegations with a statement contending that "every time violence erupts, and the police have to act, they are accused of shooting or hurting innocent children. The blame should, however, be attributed to the instigators of the violence who do not consider their actions before using children for their evil purposes."

The statement urged those alleging misconduct to file affidavits at local police stations.

Unlike yesterday, police did not prevent journalists from entering most of the riot-torn areas. But many were ordered away by rock-throwing youths, and one photographer was slightly injured by a rock.

By nightfall most of the roads had been reopened and police had pulled back their vehicles to the edges of the town.

Police reported 12 more deaths, bringing the official toll to 28 for the three days of rioting that erupted after the arrest of activist Rev. Allan Boesak and a police crackdown on an illegal but peaceful protest march called to demand the release of imprisoned black nationalist Nelson Mandela. The confirmed death toll from the past year of political violence in South Africa now stands at about 675.

At least 150 people have been reported injured in the Cape Town townships, most of them from police gunshot wounds. Hospital sources cited by the independent South African Press Association reported "gruesome" injuries from police shotgun pellets, including collapsed lungs, ruptured livers, blinded eyes and "terrible maimings."

Among the dead reported today were two Colored youths, ages 12 and 13, shot by police yesterday evening when they went outside to check on a disturbance near their house, according to Francoise Bailey, the sister of one of them.

She said her family heard shots and her mother went outside and found the bodies of the two boys.

Police reported incidents of violence in a dozen other black townships throughout South Africa. They also announced 70 more arrests under emergency powers regulations, bringing to 2,414 the number of persons detained since the emergency decree took effect nearly six weeks ago. Of those, 1,196 are still held without charge.

Foreign ministers from Italy, the Netherlands and Luxembourg arrived here today to begin a three-day mission already steeped in controversy because the ministers had agreed in advance not to "propose formulas and time schedules" for political reform here. South African officials had insisted on such a statement before allowing the ministers to come.

Two leading opponents of the system of strict segregation called apartheid, Anglican Bishop Desmond Tutu and South African Council of Churches Secretary General C. F. Beyers Naude, issued a statement expressing disappointment that the ministers had acceded to "unacceptable conditions." They said they met with the diplomats reluctantly today and urged them to recommend strong economic pressure against South Africa when they report back to European Community colleagues at a special meeting Sept. 10.

The ministers heard a very different plea from Zulu Chief Gatsha Buthelezi, who argued strongly against economic sanctions. Buthelezi, a political moderate, has been increasingly at odds with black radicals and with leaders such as Tutu, who have supported a more activist stance against the white government.

The 10 European Community countries along with Spain and Portugal have recalled their ambassadors for consultations as a sign of their disapproval of the government's state of emergency. The Sept. 10 meeting will consider joint measures against South Africa to protest apartheid and the emergency decree.

Delegates of the opposition Progressive Federal Party, meeting in Durban, today voted after an emotional four-hour debate to field candidates in elections for the new Colored and Indian parliaments. The vote underlined the classic split among liberals here over whether to use existing political institutions in opposing the government.

Proponents of the new move, led by party leader Frederik van Zyl Slabbert, argued that the party should use every peaceful tool, including parliamentary elections, to fight against apartheid. He emphasized that the party regarded the year-old constitution that established two new houses for Coloreds and Indians as "totally inadequate" because it excludes the black majority and is based on race segregation. But he said the party would use the parliaments as a base to argue for a new constitution based on the principle of one-man, one-vote.

But critics warned that fielding candidates would be seen by antiapartheid activists as a tacit endorsement of the new parliamentary system and would further alienate those activists from the party. It could especially damage relations between the party and the antiapartheid United Democratic Front, whose supporters view members of the new parliamentary bodies as sellouts and collaborators with the apartheid system.

The government in a new reform effort today announced it was extending long-term lease rights to 34 black townships in Cape Province, an area that until early this year was technically reserved for whites and Coloreds. The move allows about 152,000 blacks to qualify for 99-year leaseholds on houses.

The government said that with the extension, legal residents in most of South Africa's urban black communities can now qualify for leasing rights for their houses, which until a few years ago could only be rented for short times. The measure is a further acknowledgement by the government that some urban blacks should have permanent residential status in what had once been considered whites-only portions of South Africa.