DUBLIN, JUNE 26 -- Black nationalist leader Nelson Mandela and the African National Congress today won an important victory from the heads of state of the European Community, who decided to maintain economic sanctions against South Africa despite applauding President Frederik W. de Klerk's steps toward political reform.

Winding up a two-day summit conference here, the heads of the 12 member states said they were willing to consider a "gradual relaxation" of the punitive measures, but only "when there is further clear evidence that the process of change already initiated continues."

Their statement gave no indication of when changes in EC policy could take place, but officials here suggested moves could come sometime during the next six months, provided de Klerk continues down the path toward dismantling the apartheid system of white domination.

The statement was an endorsement of Mandela's position, reiterated during his current visit to the United States, that sanctions should not be lifted or relaxed until South Africa's white-ruled government moves much further in abolishing apartheid and instituting a one-man, one-vote system of nonracial democracy.

In the past, the Western European leaders have called for the South African government to release political prisoners, including Mandela, lift the nationwide state of emergency in force since 1986, end the ban on black nationalist movements and open negotiations with blacks over the country's political future. They also called for a commitment by the government to abolish apartheid.

Since then, de Klerk's supporters argue he has taken all of those steps, although the emergency is still in force in Natal province, scene of a violent power struggle among blacks, and a debate continues over how quickly and how thoroughly he plans to end apartheid.

A white South African observer here said the EC leaders appeared to have "moved the goal posts" by refusing to ease sanctions in response to de Klerk's actions despite praising his "foresight and courage."

{South Africa's government welcomed the EC statement, the Associated Press reported from Pretoria. "What the declaration says, albeit not in so many words, is that South Africa has regained international respectability," Foreign Minister Roelof F. Botha said. "The use of violence is rejected. By implication, this means that the ANC's adherence to the armed struggle is unacceptable."}

Some officials in the British delegation, who along with Portugal had pressed for further easing, said Ireland, which held the EC Council of Ministers presidency for the past six months, pressed to maintain sanctions in part because of Mandela's planned visit here Sunday. They said Ireland would have been publicly embarrassed if the EC had decided to take a step opposed by the black leader.

British Prime Minister Margaret Thatcher, who has already lifted Britain's ban on private investment in South Africa, said it was "a great pity" that the EC had not gone further, but she predicted that most member states would gradually ease voluntary sanctions on their own. She said members were waiting to see progress on measures to repeal laws such as the Group Areas Act, which enshrines discrimination in housing and is one of the cornerstones of apartheid.

EC leaders decided in 1986 to ban new investment and end imports of gold coins and iron and steel products from South Africa. A year earlier, they had ceased oil exports and sales of equipment to the South African police.

The measures have been largely symbolic; trade between South Africa and the EC states nearly doubled between 1985 and 1988, while voluntary restrictions on new loans by international banks concerned about South Africa's credit-worthiness have proven far more punitive. But de Klerk's government nonetheless sought relaxation of the measures to demonstrate to white South Africans that its reform program would end the country's international isolation.

The 12 European leaders also expressed support for Soviet President Mikhail Gorbachev's reform efforts and authorized an urgent study of the Soviet Union's economic needs. The wording of the statement was sufficiently vague to allow West Germany and France to claim the EC had committed itself to developing a program of financial assistance to Moscow, while Britain denied making any such pledge.

The EC leaders committed themselves to accelerate the pace of political and monetary union, scheduling conferences on both issues for December in Rome, although here too their wording was broad enough to allow for widely different views. French President Francois Mitterrand said the ultimate goal would be a "federal" Europe with a central government wielding broad powers, while Thatcher said she envisaged "12 independent, sovereign states," prepared to cooperate more closely on political and economic matters but remaining separate nations.

The leaders reiterated concern over "the lamentable position concerning the observance of human rights" in the Israeli-occupied West Bank and Gaza Strip and called on Israel to adhere to its obligations to the Palestinian population under the Geneva Convention's provisions on occupied territory. It also called for Israeli Prime Minister Yitzhak Shamir to conduct "political dialogue" with Palestinians, but stopped short of demanding Israeli recognition of the Palestine Liberation Organization or of asking the United States to resume its dialogue with the PLO.

In an environmental statement, the EC called for talks with Brazil to save the Amazon rain forests and warned: "Our planet is gravely at risk. The earth's atmosphere is seriously threatened." It urged efforts toward greater energy efficiency and said specific targets should be set for limiting gas emissions.

In an expected move, the EC reappointed European Commission President Jacques Delors to another two-year-term.