TORONTO, JUNE 18 -- Nelson Mandela, receiving a tumultuous hero's welcome in a country that has championed economic sanctions against South Africa for nearly three decades, asked Canada today to "walk the last mile" with his African National Congress in the struggle against apartheid and was assured by Canadian leaders that they would not relax international pressure on the white government in Pretoria.

While acknowledging that South African President Frederik W. de Klerk is "honestly committed" to a peaceful transformation to democratic rule, Mandela said that South African police and white extremist groups, backed by black vigilante groups who are "willing to serve their white paymasters," continue to kill and maim blacks to protect white-minority rule.

"The fact of the matter is, the apartheid system is still in place," the 71-year-old black nationalist leader told a joint session of Parliament in Ottawa before flying to Toronto. A round of enthusiastic rallies and parades here underscored the worldwide stature he has acquired during the first half of his six-week tour of Europe and North America.

At an appearance outside the Ontario provincial legislature, Mandela told the crowd:

"We are at the threshold of major changes in South Africa. We are confident that victory is in sight, but as in a steeplechase race, the last hurdles are the most difficult to overcome."

Reiterating that sanctions must be retained, he cited the need for South Africa to release all political detainees, end political trials, return all political exiles and repeal all remaining apartheid laws.

Canada's minister for external affairs, Joe Clark, assured Mandela: "It would be wrong now to relax those pressures. We have stood with you in your own remarkable walk to freedom, and we intend to be with you along that walk until the people of South Africa are free."

At a news conference earlier, when asked whether he thought a democratically elected government in South Africa could be negotiated as early as next year, Mandela said he was optimistic as long as economic sanctions are maintained.

Mandela's half-hour address to Parliament -- the first in 40 years by anyone other than a head of state -- seemed to be directed as much to the European Community leaders who will meet June 25 in Dublin to consider lifting sanctions as it was to Canadians.

In welcoming the ANC leader, Prime Minister Brian Mulroney said that it was the late prime minister John Diefenbaker's anti-apartheid stand at the 1961 conference of Commonwealth nations that prompted South Africa to withdraw from the Commonwealth, marking the beginning of the intensification of international pressure on South Africa.

"The government of Canada believes the way to advance the process of democracy in South Africa is to maintain the existence of sanctions," Mulroney declared. "You can count on Canada in the months and years ahead," he said, although he made clear that support for the ANC would continue to come from nongovernmental organizations.

In his news conference, Mandela acknowledged British Prime Minister Margaret Thatcher's potential role in supporting an easing of sanctions at the EC summit but said that during his European tour he had received assurances that "sanctions will be supported by an overwhelming majority of heads of governments."

When asked if he would ask President Bush during his visit to the United States to try to influence Thatcher, Mandela replied: "We believe that we are in as good or better position as any government to influence the views of Prime Minister Margaret Thatcher."

He said he could not forecast a response in Washington to his call for increased sanctions but that he has "reason to believe" his position will be supported by Congress and the administration. "I have no reason to be pessimistic about their response," Mandela said.

Mandela also underlined another key purpose of his foreign tour -- to request "material assistance" for the repatriation and resettlement of ANC exiles and "to help us in our political work."

The ANC leader sidestepped a question at his news conference about published reports that the U.S. Central Intelligence Agency played a key role in his arrest in South Africa in 1962 by informing the police of his whereabouts. "Let us let bygones be bygones. Let us consider the present and the future," Mandela said.