Prime Minister Margaret Thatcher fended off heated opposition criticism today in the third week of intense debate over her stand against South African sanctions, telling Parliament that sanctions are immoral and would cause unemployment not only in southern Africa but also in Britain.

"I don't take lectures," Thatcher responded sharply when the opposition Labor Party leader, Neil Kinnock, charged that her antisanctions policy helped contribute to the deaths of black children in South Africa.

Among the many outside observers at today's particularly raucous session of the regular Tuesday "Prime Minister's Question Time" in the House of Commons was Soviet Foreign Minister Eduard Shevardnadze, here on a two-day official visit.

Shevardnadze looked alternately bemused and baffled as members of Parliament hurled insults at Thatcher and tried to drown out her presentation with catcalls and boos. Seated in the distinguished visitor's gallery in front of a row of tourists, he slouched back with arms crossed, then leaned over the railing to peer down at the packed chamber, occasionally smiling and shaking his head as his interpreter struggled visibly to translate democracy at work below.

A loyal member of Thatcher's Conservative Party attempted to insert a measured question, apparently for Shevardnadze's benefit, on possibilities for a summit this year between President Reagan and Soviet leader Mikhail Gorbachev.

But no sooner had Thatcher acknowledged hope for such a meeting than the subject was quickly submerged under a new onslaught of questions about sanctions.

The sanctions issue has dominated political debate here for weeks, with Labor seeking political advantage and moral high ground by painting Thatcher as heartless, intransigent and isolated from Britain's Commonwealth allies.

Thatcher has countered that Labor, which itself opposed sanctions against South Africa when it ran the government here during the mid-1970s, has no business lecturing her.

Although her Cabinet is reportedly divided on the issue, Thatcher has gained support from numerous backbench Conservatives and a significant portion of the British business community.

The largest single source of foreign investment in South Africa, Britain has more than $14 billion invested directly or indirectly in South Africa. Britain is also one of South Africa's major trading partners: in 1984, total trade between the two countries was more than $2.5 billion.

With persistently high unemployment here, Thatcher has argued that as many as 120,000 British jobs could be lost if she agreed to comprehensive economic sanctions against Pretoria.

Pressure to change her stand against sanctions seems only to have stiffened Thatcher's convictions.

In a series of media interviews in recent weeks, she has dismissed arguments that most South African blacks favor sanctions as misleading and implied that those speaking for them, such as black Anglican Bishop Desmond Tutu, are not as representative of black opinion as is often claimed.

She has questioned the "morality" of the prosanctions argument, saying that the economic consequences would fall most heavily on the black population, not only in South Africa but in neighboring black-ruled states.

While deploring apartheid as "repugnant," Thatcher has said that the Pretoria government has moved to dismantle many aspects of it, and can be persuaded to move further.

Asked today to express concern over this week's withdrawal of five African nations from the Commonwealth Games, scheduled to begin a week from Thursday in Edinburgh, Scotland, Thatcher described the games boycott as regrettable and "unjust."

The five nations -- Nigeria, Ghana, Kenya, Uganda and Tanzania -- are protesting Britain's refusal to go along with the vast majority of the Commonwealth's 49 member nations in seeking coordinated and comprehensive sanctions against South Africa.

India, whose delegation is traditionally one of the largest at the quadrennial sporting event, has delayed the departure of its team to Edinburgh. Indian officials said they will decide whether their athletes will attend after a meeting of African sporting officials Friday in Zimbabwe to discuss possible additional boycotts.

Botswana, Swaziland and Gambia have said they will attend the games. In a speech here today, Commonwealth Secretary General Shridath Ramphal called on all sides of the boycott debate to seek compromise. He urged those favoring a boycott to "keep their options open," and called on Britain to make some concession on the sanctions issue that would allow them to attend.

Whatever happens at the games, the Commonwealth conflict is expected to continue early next month, when the heads of government of a special seven-nation committee on South Africa, including Britain, are due to meet here to consider the sanctions issue.

The sanctions argument within the House of Commons will resume Wednesday during a full-scale debate called by Labor. Presenting the government's case will be Foreign Secretary Geoffrey Howe, who last week undertook the first stage of a "last ditch" negotiating effort in southern Africa under the auspices of the European Community.

Zambian President Kenneth Kaunda and Zimbabwe President Robert Mugabe told Howe that they saw no further point in trying to achieve a diplomatic solution to South Africa's racial conflict.

Howe is scheduled to travel to South Africa for two separate meetings with President P. W. Botha before the end of July. Many black leaders there have said they will refuse to meet with Howe.

Following his attendance at today's parliamentary session, Shevardnadze held talks with Kinnock and other opposition leaders. Since arriving in London Sunday evening, Shevardnadze has had two lengthy meetings with Howe and one with Thatcher -- to whom he extended an invitation to visit Gorbachev in Moscow next year.

Shevardnadze plans to hold a news conference here before departing Wednesday morning.