President Reagan, engaged in what he termed "a little self-defense" against his critics, said today that his administration has not cut the federal education budget and that "each budget is still bigger than the last one."

Although he didn't identify the critics, before Reagan spoke here one of the leading Democratic presidential candidates, former vice president Walter F. Mondale, accused Reagan of pursuing an education policy that is "not to spend any money on education." Mondale, at an earlier separate appearance at the University of Minnesota, said he would not "let Reagan whitewash his own record."

Reagan came to the Twin Cities to push his program for education, which is emerging as a major issue, and for a fund-raiser for Sen. Rudy Boschwitz (R-Minn.), who is up for reelection next year. He was heckled on the streets by several thousand demonstrators protesting his tax cuts and nuclear arms and education policies.

Appearing in the gymnasium of a high school that was closed last year partly because of budget cuts, Reagan told a regional panel of the Commission on Excellence in Education that "the amount of money in education today" is greater than the national defense budget. In April the commission released a critical report on the state of American schools.

"In 1982 the total budget for national defense was $179 billion," he told an audience of about 1,000. "It was $215 billion for education. And I don't fault that at all. Education is truly important, and as important to our national security as defense."

Deputy White House press secretary Larry Speakes said the president was referring to "all spending" on education-related programs and to the "volume of student loans, not necessarily federal expenditures," in the budget numbers he cited.

According to figures in the president's last budget, funding for educational purposes has fallen from $14.3 billion in fiscal 1982 to an estimated $13.5 billion in fiscal 1984. In addition, the administration proposed cutting funds for higher education programs from $6.5 billion to $6.3 billion. The proposal was stymied by Congress.

Funding for elementary, secondary and vocational education dropped from $6.7 billion in 1982 to an estimated $6.5 billion in 1983, and guaranteed student loans fell from $3 billion in 1982 to an estimated $2.2 billion in 1983.

"The truth of the matter is we haven't cut any budgets," Reagan said, in direct contradiction to the numbers. "What we've done is reduce the proposed increases in the budget. But each budget is still bigger than the last one."

The president then spoke of his experience in paying for schooling.

"I have had students accost me and tell me that we have reduced their ability to get help. Well, for a fellow that worked his own way through school, I understand the problem of students that have to. I must say it wasn't too arduous. I washed dishes in a girls' dormitory."

Reagan then cited figures for several education programs that again differed significantly from official budget numbers.

One panelist told Reagan that a committee that met earlier suggested that the United States "put one last missile in the Midwest and spend that money on education." Reagan did not laugh or respond to the comment.

The president seemed to drift in his thoughts during the panel discussion. At one point he asked Education Secretary T.H. Bell to announce what steps the administration is taking to respond to the national commission's report, entitled "A Nation at Risk."

Bell told of a plan to begin a major effort to "help the huge number of adult illiterates we have in this country." Bell also reminded Reagan that in his State of the Union speech he had recommended a block grant to aid students in math and sciences.

Bell, who like Reagan took off his jacket in the hot school gym, said the student aid program also is being reviewed to see if "we can rewrite it to . . . encourage more gifted and talented and promising young people to move into teaching."

Reagan has made merit pay for teachers the linchpin of his program for improving schools, much to the distress of many teachers groups, including the National Education Association--a group closely tied to Mondale. The president said today that concerns that administrators will not be able to judge teachers fairly and accurately are baseless.

"Teachers who grade students ought to be able to grade each other," Reagan said. He added that if extra pay is needed to reward the best teachers it can come from local school boards who can make paying the best teachers their first priority.

Reagan's trip here is the first of several dedicated to the issue of public education. He is scheduled to travel to Tennessee and New Mexico next week and to speak on behalf of merit pay for teachers.