Former vice president Walter Mondale yesterday accused the Reagan administration of demonstrating an "icy indifference to human need and justice" by singling out the poor and defenseless to bear the brunt of its social and economic policies.

In a no-holds-barred speech to the National Urban League convention here, Mondale said that President Reagan had delayed a decision on extending the Voting Rights Act because his aides were studying ways to weaken it. Mondale said the administration had left numerous civil rights positions in the government unfilled because it does not care about enforcing equal opportunity laws.

"It is not only what they are doing that offends a decent sense of fairness," Mondale said. "It's how they are doing it. Someone told me the other day he thought it might be necessary to cut money for handicapped children, but at least our leaders shouldn't look like they are enjoying it."

Urban League delegates, who listened politely, if warily, this week to a defense of administration policies by Vice President Bush and a parade of Cabinet members, rose three times yesterday to give Mondale cheering, standing ovations.

Mondale clearly enjoyed both the delegates' warm, enthusiastic response and his own attacks on the Reagan White House. When he arrived on the podium, he greeted League President Vernon Hordan by calling him "Mr. Mayor," a reference to a recent White House meeting in which Reagan mistook Housing and Urban Development Secretary Samuel R, Pierce Jr., the only black member of the Cabinet, for a mayor.

Later, Jordan attacked administration spokesmen for blurring the "real issues" in their speeches to the League and for "repeating the tired slogans we heard before."

Jordan said they had failed to answer his question of how poor and black people were supposed to endure in the "interregnum" after cuts in social programs take effect and before the promised prosperity returns.

Giving every sign of contemplating a new run for the presidency, Mondale reminded the delegates of his role in the Great Society and in civil rights struggles, which he called "the most successful, peaceful revolution for human and social justice in the history of humanity."

The former vice president disagreed with Bush and others that the social programs of the last two decades had not worked, although he acknowledged that they had been hurt by instances of poor administration, waste and theft.

But, he said, "we started to arm people with the tools to compete in a less discriminatory America: good education, decent health care, better housing, programs to rebuild the cities and strengthen our farms, economic development programs and minority enterprise programs, and many others."

He noted that high school dropout rates for blacks had been reduced sharply in the last two decades, the number of blacks attending college and vocational schools has doubled, and comapnies not even founded 12 years ago now are among the top 100 black-owned corporations.

"We achieved so much as a people because we have listened so closely to our conscience," Mondale said. "We've been true to our history because we led with our values. What worries me today is that just when America is hitting its moral stride, there are those who would stop our progress dead in its tracks."

Mondale skipped over an entire section of his prepared text in which he suggested that the federal deficit be restrained, stifling regulations lifted, wasteful programs be made better and tax relief be granted to those "over-taxes."

But he criticized the Reagan tax cuts, saying they will cause unacceptably high deficits and keep interest rates high for a long time.

"The tragedy is that we could do what needs to be done -- tighten the budget and reduce our deficits -- and we could do it fairly," Mondale said. "Our needed programs would have to be constrained with the others, but they could go forward."