Walter F. Mondale today led the Democratic Party's fight to retain some of its most cherished political legacies -- and its pride -- before a crowd of 50,000 in Boston Common, a hub of the party's liberal tradition.

"I come to you today as a people's Democrat, as a full-employment Democrat in the tradition of Roosevelt and Truman and Kennedy and Humphrey," said Mondale, who was flanked by Sen. Edward M. Kennedy, House Speaker Thomas P. (Tip) O'Neill Jr., Gov. Michael S. Dukakis and other prominent symbols of the party's left wing.

Claiming a crowd 10 times larger than the one that turned out for President Reagan a few blocks away on Thursday, O'Neill said: "We haven't forgotten compassion in this country . . . . It was the heart and soul of America. We made America, middle-class America, and let's keep it. Let's not go back to the . . . '30s."

Edward Kennedy challenged Reagan's invocation of the John F. Kennedy legacy to woo Democratic supporters. Reagan and Ray Shamie, the Republican nominee for Senate, did so at Thursday's rally outside Boston City Hall.

"I only wish they stood in the clear light of the principles in which John F. Kennedy believed . . . fairness . . . justice and progress toward peace," Kennedy said.

He quoted his brother as saying that the 1960 campaign was "a race between the comfortable and the concerned," and added, "The stakes in this 1984 campaign are very much the same."

In fact, the stakes may be even higher for Mondale, his campaign and the party as they struggle to stave off a Reagan landslide projected by most public opinion polls with more than Dukakis' claim today that "something's happening out there that the pollsters just aren't picking up."

Massachusetts is traditionally one of the nation's most liberal states and a Democratic stronghold. Yet Mondale trails Reagan here as the Democratic nominee scrambles to gain ground in the campaign's final days.

Indeed, it was ironic to find Mondale, a self-proclaimed Democrats' Democrat, campaigning today here and in West Virginia, two states that have voted Democratic in five of the last six presidential elections.

Campaign aides insist that Mondale's is a "270-electoral-vote" strategy aimed at winning the presidency on Tuesday. Yet victories in such relatively marginal states as Massachusetts (13 electoral votes) and West Virginia (six electoral votes) would probably do more to prevent a Republican rout than prompt a Democratic conquest.

In addition to Kennedy, Dukakis and O'Neill, Mondale received an introduction today from folk singers Peter Yarrow and Mary Travers, who led the audience in "This Land Is Your Land," an anthem of the early and middle '60s when Democrats controlled Congress and the White House.

Mondale continued to attack the administration as being indifferent to the underprivileged. Under Reagan, he said, "If you're unemployed, it's too bad. If you're old, it's tough luck. If you're sick, it's good luck. If you're black or Hispanic, you're out of luck. And if you're handicapped, you shouldn't be."

He also criticized the administration's tentative proposals to increase taxes on disability and workman's compensation benefits and to eliminate federal tax deductions for payment of state and local taxes.

"I say in this decent country, that is wrong," Mondale said to rising applause. "Middle- and moderate-income Americans deserve some help, some protection, and let the wealthy and the big corporations and the loophole-users join the rest of us in paying for this blessed society."

It essentially was the same message he delivered earlier in the day at the Admiral King High School in Lorain, Ohio. There, Mondale listed a litany of economic hardships that he said Reaganomics has created for steelworkers and dockworkers in the area.

"On every one of these issues that affects your life, the president is out to lunch," Mondale told a cheering crowd of about 5,000.

At a crowded sunset rally in the public square of Wilkes-Barre, Pa., Mondale said, "Reagan is the worst president for working Americans since Hoover." Occasionally shouting over the taunts of hecklers, Mondale told a shivering crowd of about 7,000: "When you're unemployed, let's have a president who doesn't sleep 'till you're back to work." Mondale then flew to Charleston, W.Va., for a rally.