LONDON, JULY 4 -- For months, black nationalist leader Nelson Mandela and British Prime Minister Margaret Thatcher had been sparring with each other across two continents and the equator over some of the most critical issues facing South Africa. But when they finally met today, the two leaders emphasized the positive, saying they found much to agree on and expressing deep admiration for one another.

Mandela, whom admirers call the father of the nation, met Thatcher, frequently referred to as the Iron Lady, for three hours at Downing Street. Both sides described the session as "warm and cordial." As for the main issues that divide them -- economic sanctions against South Africa and the renunciation of violence by blacks there -- the two leaders listened to each other's presentations and agreed to disagree.

When it was over, Mandela praised Thatcher's "integrity and sincerity" and called her "an enemy of apartheid," South Africa's system of domination by the white minority.

Thatcher, in turn, said she was impressed by Mandela's determination to bring an end to violence and his awareness of the need to start negotiations between black leaders and the white government, according to her aides.

It was a far cry from the debate that raged between the two leaders in the months since Mandela's release from prison last February. Since then, Thatcher -- who in 1987 branded Mandela's African National Congress a "terrorist organization," has been pressing the United States and the European Community to lift their sanctions on Pretoria to reward South African President Frederik W. de Klerk for his willingness to push ahead with profound change despite intense opposition from some whites.

Mandela has insisted in the past that sanctions should remain until South Africa had moved "irreversibly" by ending apartheid and instituting a democratic system of majority rule. He appeared to modify that stance just slightly today, telling British business leaders this morning that he believed rapid progress was possible once negotiations began between the ANC and the government.

If such progress were achieved, Mandela suggested he might call for the easing of sanctions before the end of the year. He also told the businessmen that he hoped someday to meet Thatcher together with de Klerk.

The black leader also gave ground by effectively retracting comments he made Monday in Dublin calling for discussions between Britain and the Irish Republican Army, which is waging a violent struggle to end British rule in Northern Ireland.

The remarks caused outrage across a broad political spectrum here, with even Labor Party leader Neil Kinnock, one of Mandela's strongest supporters, saying the black leader had been mistaken. By Monday evening, Mandela was saying he had meant only to condemn "mutual slaughter" but denied he sought to "prescribe to anyone as to how this should be achieved."

Some observers attributed the slip to Mandela's apparent state of fatigue. Today's visit to Downing Street, followed by a meeting with Kinnock and a final press conference, wrapped up a four-week whirlwind tour of the United States and Europe in which the 72-year-old Mandela has been feted and honored while putting himself through a grueling schedule that left his younger aides exhausted.

Showing signs of fatigue, Mandela yesterday cut short several meetings and skipped an evening rally. His pace today was slow and deliberate, and he looked more rested as he stepped from a car into the rain to meet Thatcher at the door of 10 Downing Street.

Afterward, both sides talked about the warmth of the meeting and the "great personal rapport" that Thatcher's aides noted between the two leaders. Thatcher, they said, was particularly impressed by Mandela's "gentlemanly fashion."

Mandela told reporters he and Thatcher shared "a common approach which we can use in order to seek solutions," adding, however, that "there were a number of points on which we did not see eye to eye completely."

Despite economic sanctions, more than 200 British companies are doing business in South Africa, with investments worth about $5 billion.

Mandela also met today with Rockefeller Foundation President Peter Goldmark to pursue the possiblity of an international development bank for a post-apartheid South Africa.

The bank would be similar to the European Development Bank for Eastern Europe and be backed by a combination of governments and private investors.

Washington Post staff writer Steven J. Mufson contributed to this report.