Prime Minister Margaret Thatcher's Conservatives won a landslide victory yesterday in an election that also marked the worst defeat for the opposition Labor Party in 60 years.

The new centrist Alliance of Liberals and Social Democrats won about a quarter of the popular vote but actually took fewer seats than it had previously held because of Britain's system of apportioning places in Parliament.

Among the prominent losers were Tony Benn, leader of Labor's left wing who had served in Parliament for 33 years, and two of the four founders of the Social Democrats, Shirley Williams and William Rodgers. Roy Jenkins and David Owen, the Social Democrats' other leaders, were among the party's only four winners as counting passed the two-thirds mark.

Thatcher thus becomes the first Conservative prime minister in this century to win two consecutive elections. Her parliamentary majority--estimated to be about 130 seats over the combined opposition--would also be the largest of any party in modern times.

But the results of the election are by no means an unequivocal endorsement of Thatcher and her policies. A majority of the nation's voters chose other parties. However, popular vote totals do not determine the number of seats because each constituency is a separate contest and only winning counts.

According to the unofficial returns, the popular vote was split 42 percent for the Conservatives, 29.9 percent for Labor, 25.6 percent for the Alliance. Other small parties divided the rest.

Although Labor still will have by far the second-largest number of seats in Parliament, its popular vote total dropped substantially from the last election in 1979. Most of those votes went to the Alliance, which almost doubled the number that the Liberals alone received four years ago. Many of the Social Democratic members of the Alliance were originally elected as Labor Party members but defected to form a new party in 1981 when they became dissatisfied with Labor's increasingly leftist policies.

Despite their huge margin in the House of Commons, the Conservatives share of the popular vote count was down slightly from what it was four years ago.

These disparities between votes and parliamentary seats are certain to provide new impetus for the debate over whether Britain should adopt the system of proportional representation used in many other parliamentary democracies.

The improved showing of the Liberals, in particular, establishes them as a significant third force in British politics. The future of the Social Democrats is in doubt, however. Most of its parliamentary members were defeated in yesterday's vote.

Thatcher said the Conservatives had won a "historic victory" and received a mandate for continuing the economic strategies of her first term. She is expected to announce her new Cabinet quickly.

Labor leader Michael Foot, in conceding defeat, blamed the Alliance, which, he said, "has assisted in giving victory to a government I believe the country does not wish to see running our affairs over the next five years." He added, "Of course, I must accept my responsibility for this affair. . . . It's a tragedy."

Labor's poor showing prompted immediate speculation about the replacement of the 69-year-old Foot as party leader. Should Foot go, a new leadership will face the considerable task of attracting center-left voters back from the Alliance.

Liberal leader David Steel said late last night that it was too late for Labor to revive its former position. "We're seeing the end of the Labor Party as the major opposition to the Conservatives," he said. "To that extent we have broken the mold."

Voting was brisk throughout the British Isles in generally good weather. The turnout was expected to be about 76 percent.

Thatcher, Steel and Jenkins cast their ballots early yesterday. Foot voted by mail. Final results will not be available until midday today.

When Thatcher dissolved Parliament in May to call the elections, the Conservatives had a 35-seat majority with 334 seats. Labor had 239 seats, and the Alliance had 42. The rest was divided among other small parties.

Yesterday's voting capped one of the bitterest campaigns in modern British history. From the outset, Thatcher's commanding lead in the opinion polls meant that the opposition parties had to overcome her advantages of being the front-runner as well as the incumbent.

The parties were deeply divided on policy issues. Thatcher's success in reducing inflation from a peak in 1980 of 21.9 percent to about 4 percent currently allowed her to claim that the Tory economic strategy was working.

Labor said that only a massive infusion of government spending would revive the economy and restore a substantial number of the millions of jobs lost in recent years.

The Alliance position combined a moderate reflation through government spending with an incomes policy to keep inflation under control.

On defense, the Conservatives have increased spending on conventional arms and are pledged to modernize Britain's nuclear forces through acquisition of American-made Trident D5 submarine-launched missiles. Although urging a successful conclusion to the nuclear arms reduction talks in Geneva, Thatcher's government has never wavered from its pledge to deploy medium-range cruise missiles in Britain later this year if the U.S.-Soviet negotiations fail.

Labor, although split on the particulars, put forth a "nonnuclear" defense policy including immediate cancellation of the cruise deployment and purchase of the Trident missiles. Foot joined the party's left wing in supporting the expulsion of all American bases from Britain.

The Alliance also called for canceling Trident.