In a surprise move amid a heated struggle for leadership of Japan's ruling party, Prime Minister Zenko Suzuki today announced that he will step down as president of the Liberal Democratic Party, a post that carries with it the premiership.

Suzuki, 71, said in a statement issued late this afternoon that he would resign "to sweep away the bad faith and hard feelings" touched off by intensifying feuding among key factions in advance of elections for party president scheduled for Nov. 26.

The abrupt announcement stunned political analysts here, who had considered Suzuki the odds-on favorite in the race despite mounting criticism within party ranks of his 27-month-old administration. It sent top party bosses into a round of hasty behind-the scenes negotiations to select a possible successor from among prominent party members.

Yasuhiro Nakasone, 64, a member of the Suzuki Cabinet, has been touted in the Japanese press as a strong candidate to succeed Suzuki. But political analysis here caution that Nakasone, or any of the handful of other possible successors, are now far from commanding the broad support of party elders needed to get the job.

No significant changes in Japan's foreign or domestic policies were expected as a result of the shake-up in the conservative party, which has run the government here for the past 27 years.

It was not immediately clear, however, how or when a successor would be chosen or when Suzuki's resignation would take effect. Suzuki, who had been expected to make his candidacy official soon, said today he decided instead to step aside to open the door to "fresh, new leadership" in a bid to restore badly strained party unity.

According to political analysts here, however, the move may well aggravate the bickering, as party faction leaders vie to strengthen their clout. But there is little danger of the business-oriented, pro-American party loosing its grip of the Diet, the Japansese parliament, where it commands absolute majorities in both houses. The party now holds 287 of the 511 seats in the lower house where most key Diet business is done.

Suzuki has come under blistering attack in recent months by the party's so-called "anti-mainstream" forces, a group of factions that controls roughly one-third of the party's strength in the Diet. While lacking enough votes to challenge Suzuki's rule directly, they have castigated him for his alleged inability to prune Japan's bulging government deficits and to help spark recovery of the country's increasingly sluggish economy.

Leading the onslaught have been Toshio Komoto, 71, and Shintaro Abe, 58, two key Cabinet economic ministers who have been widely regarded as potential front-runners in next month's elections for party president. Another "anti-mainstream" leader, Ichiro Nakagawa, 57, who also serves in the Suzuki Cabinet, is as yet the only publicly declared candidate.

By suddenly relinquishing power, Suzuki has forced the party's mainstream leadership, to which he belongs, to recalculate quickly its own election strategies. The political equation has been complicated, however, by the position of former prime minister Kakuei Tanaka, who is widely reputed to have helped install Suzuki in office in July 1980.

Tanaka, who is currently standing trial for his alleged involvement in the Lockheed bribery scandal, now leads the party's largest faction, despite the fact that he was forced to give up official party membership following his indictment in the case.

In the past, the powerful kingmaker has favored Nakasone, who now heads the smallest faction in the party mainstream, as a possible candidate for the party's top job. Nakasone serves in a Cabinet-level post as director general of the Administrative Management Ageny.

Political analysts here say, however, that it is still too early to predict behind whom Tanaka will throw his weight, although some suggest that he may be considering a senior politician more closely aligned with his own faction. Suzuki, whose own faction ranks second only to Tanaka's in size, is also expected to have an important say in selecting his sucessor because of what is widely viewed here as his statesmanlike decision to resign in the best interests of the party.

Analysts have not ruled out the possibility that former prime minister Takeo Fukuda, who was ousted from the office in 1978, might attempt to turn the current party turmoil to his advantage and stage a political comeback. At 77, Fukuda remains a vigorous, shrewd party veteran with a large following in the anti-mainstream camp.

Tonight, top party officials representing all factions met to discuss the selection of Suzuki's sucessor. If intra-party negotiations, which are expected to continue for the rest of the week, go smoothly, a new party boss, and hence prime minister, might be chosen in a matter of days, observers here said. This would relegate next month's party elections to a purely ceremonial function to rubber-stamp the senior party officials' choice.

Failure to decide on a suitable compromise candidate, however, could make the November runoff a hotly contested political showdown.

Suzuki came to power in 1980 following the sudden death of then-prime minister Masayoshi Ohira and amid bruising factional struggles that threatened to tear the party apart and end its monopoly on power.

Plucked from outside the party's circle of political intimates, Suzuki was originally considered an interim caretaker to run the government until party leaders could bury the hatchet and select a more prominent figure.

Although viewed as a deft political tactician and capable negotiator, Suzuki suffered from lack of expertise in foreign and domestic economic affairs and from a modest, amiable personality that left him open to attack by opponents, who charged him with failing to exert a strong enough leadership role.

To the surprise of his critics, however, he did manage to roll up a list of political credits, including the first major reforms in Japan's election system since the U.S. occupation in the early postwar period.