Former vice president Walter F. Mondale, using the rhetoric of liberalism tempered with time, officially launched his presidential candidacy here today by declaring, "I am ready to be president."

Mondale, who learned politics from the late Hubert H. Humphrey, served as Democratic state attorney general and U.S. senator from Minnesota and later was Jimmy Carter's vice president, announced his candidacy from the ornate House chamber of the Minnesota state capitol building.

He then took his large traveling party to the Iron Range of northern Minnesota to hear firsthand of the economic suffering that exists in America today and to attack the administration of President Reagan.

"At the very core of my being and at the center of my campaign and at the center of my administration is going to be a philosophy that is entirely different from the one that's causing you to suffer," he told a town meeting in Virginia, Minn. "I don't believe America was meant to be a jungle where we have survival of the fittest or survival of the richest."

Later, pledging to restore the economic might of a region with an official unemployment rate around 30 percent and pockets ranging up to 80 percent, Mondale said, "Some people say we can't do it. But this is the nation that rebuilt Europe. Ours is the nation that rebuilt Japan. And today, I think the time has come to rebuild the United States of America."

Mondale became the third official candidate for the 1984 Democratic nomination and polls give him a clear lead over his likely competitors. With a January Gallup poll giving him a 12-point lead in a head-to-head matchup with President Reagan, he begins his campaign with considerable self-confidence.

Calling himself his principal competition, Mondale said in a recent interview, "I'm not worried about the other candidates, so long as I do my job well. I think Americans are looking to see who will make the best candidate, who makes the most sense, who shows the capacity and experience needed for leadership. If I'm able to demonstrate that, I believe I'm going to be nominated and elected."

Mondale said the advantage of being the front-runner is the ability to be heard as he begins to define himself; today, before roughly 300 friends and contributors from across the country, including Humphrey's widow Muriel who has since remarried, he began that process.

After a long apprenticeship in the shadows of other politicians and with the memory of his decision to quit the 1976 presidential race, Mondale made clear that this time is different.

"I have the experience," he said. "I know where the talent is. I know the White House. I know how to shape a government. I know how to manage. I know how to defend this country. I know how to search for peace. I know who our friends are. I'm on to our enemies. I know the American people. And I know myself: I am ready. I am ready to be president of the United States."

Mondale set himself in contrast to Reagan by dismissing the notion that government is the problem facing the country.

"Human suffering, a faltering economy, a dangerous arms race, a divided America: these are the problems," he said. "Today there are American families sleeping in cars, searching for work, and tasting the grapes of wrath."

His biggest applause came when he declared his determination to reduce the "mindless, wasteful madness" of the nuclear arms race and negotiate a mutual and verifiable nuclear freeze, and later when he called for an Equal Rights Amendment to the Constitution.

Mondale said he wanted an America "where working people don't have to pay more so that the privileged can pay less."

And he urged Congress to reduce the federal deficit by scaling back defense spending and repealing tax indexing and tax cuts "for the wealthy."

He condemned the role of money in politics, reiterating that he will accept no contributions from political action committees and calling for public financing of congressional elections.

"I think it is time to declare that the government of the United States is not up for sale," he said.

But there were other notes in Mondale's speech than those of the liberalism that has marked his public career.

"In the years ahead," he said, "everything will depend on economic growth: our jobs, our defense, our commitment to social justice."

He called for stronger families, tougher discipline, cooperation between labor and management, education for basic skills, and a commitment that "convicted criminals go to jail again."

He invoked the value of community, drawing on his roots as a minister's son in southern Minnesota. And later, in the Iron Range, he visited a privately funded "food shelf" in Gilbert, Minn., that provides food for needy families.

Looking to the future, he called for a policy to strengthen entrepreneurship and free enterprise, a commitment to the sciences and a trade policy that "insists that our trading partners open their markets as wide to us as we open ours to them."

The trip to the Mesabi Range was an insight into Mondale's sense of America, as he heard, in a well-choreographed performance, from a high school student, an elderly man, a young mother and an unemployed worker who told the human side of the ravaged economy.

They were individuals, but they were symbols too, and also representative of the kind of constituency politics he has practiced so long in the Democratic Party.

Mondale has been criticized for trying too hard to satisfy those many constituencies, but in an interview, he rejected that criticism.

"There's a new theory around that the way to get elected to government is to offend everybody who would like to be your friend," he said. "I don't buy that. If you're captive, you shouldn't be president. But I'm not captive."