Japan's Liberal Democratic Party led by Prime Minister Yasuhiro Nakasone scored a decisive victory at the polls yesterday in a successful bid to strengthen its comfortable majority in the parliament's uppper house.

Late returns this morning in the Japanese media gave the largely pro-American, conservative party 66 of the 116 seats decided, a showing in line with earlier voter surveys that predicted a big Liberal Democratic Party win.

A final vote count for all 126 contested seats, half of the 252-seat House of Councillors, the upper house of the Diet, was to be announced this evening. With almost all of the votes tallied, the Liberal Democratic Party had easily retained its 65 seats up for reelection, assuring the party of continued control of all upper house committees.

Commenting on the partial results, Liberal Democratic Party Secretary General Susumu Nikaido said, "It is now certain that we will be able to maintain a stable force" in national politics where the party has frequently traded on its reputation for ensuring social and economic stability.

In contrast to the self-effacing style of previous Japanese leaders, Nakasone in his seven months in office has fostered the image of a man of action determined to tackle sticky domestic issues, including the overhauling of the government bureaucracy and tax reforms, while committing Japan to a larger role in global security affairs.

According to the latest public opinion polls, Nakasone has now regained a solid standing among voters after his popularity slumped sharply earlier this year in the wake of his endorsement of a militarily stronger Japan and bigger defense budgets. The Liberal Democratic victory, analysts said, would bolster Nakasone's ability to carry out his pro-Western policies, which feature stronger defense and economic ties with the United States.

Late returns available this morning gave the Socialists 21 seats, with the Komeito (Clean Government Party) winning 13 and the Democratic Socialists and Communists 5 each. The New Liberal Club, a Liberal Democratic Party splinter, had received only one seat.

Two tiny, single issue groups, the Salaried Workers' and Welfare parties, had won one seat each with independents gaining three seats. It was the first time that any of a dozen new "mini parties" had won seats in parliament.

Election analysts appearing on nationwide television said the vote count could show a net loss of seats for the Socialists, the country's largest opposition party, with the New Liberal Club also suffering a sharp setback. Komeito, the Democratic Socialists and the Communists were expected to hold their own or add a small number of seats.

At the Williamsburg economic summit in May, Nakasone's strong support of President Reagan's position in nuclear arms reduction talks with the Soviet Union churned up charges among Japanese opposition politicians that his hawkish stand on security affairs was leading Japan in dangerous new directions. But Nakasone's prominent, statesmanlike role in the talks, stressing broad cooperation within the western alliance, earned him points with rank-and-file voters at home, analysts said.

Combined with his new emphasis on domestic affairs, political commentator Rei Shiratori said, Nakasone's performance at Williamsburg "has allowed him to accustom voters to the need for a gradually bigger Japanese role in world affairs," while allaying fears of any abrupt shifts in national policy. Yomiuri Shimbun, a major daily newspaper, said last week that Nakasone's public support had risen to 45 percent, five points higher than when he took office.

Cloudy, unseasonably cool weather yesterday contributed to one of the smallest voter turnouts in postwar history, with only about 57 percent of eligible voters going to the polls. Another factor in the poor turnout, analysts said, may have had to do with an apparent perception among the electorate that the outcome would have practically no impact on the Liberal Democratic hold on power.

The party, which has run the government here since 1955, commands a 285-seat majority in the 511-member House of Representatives, the stronger lower house of parliament where the bulk of key Diet business is done.

Although the upper house possesses little legislative authority, its elections are regarded as an indicator of future voting patterns for the lower house, from among whose members the prime minister is chosen. The House of Representatives has one more year left to run in a four-year term before elections must be held.

Results from yesterday's elections will also determine whether ex-prime minister Kakuei Tanaka will prove successful in trying to bolster his party power base. Tanaka, a powerful Liberal Democratic kingmaker and key Nakasone supporter, is expected to be judged by a Tokyo district court this fall on charges of accepting $2.1 million in bribes from the Lockheed Aircraft Corp. while he was in office in the early 1970s.

By maneuvering his followers into choice campaign positions, Tanaka lieutenants hoped to ensure the redoutable politician's political longevity by adding to his factional strength in parliament in advance of the verdict, which is widely expected to go against him.

Out of 21 Tanaka candidates who ran in local constituencies, 14 had been assured of upper house seats, although late returns showed Tanaka supporter running less strongly thatn anticipated.

Tanaka, who was forced to resign official party membership because of his involvement in the bribery scandal, runs the party's largest Diet faction by proxy.

Tanaka's support is believed to have been instrumental in Nakasone's bid for the prime ministership. In his first days in office, Nakasone sparked a public outcry when he appointed six Tanaka followers to key positions in his 20-member Cabinet.