Warning that "we are in the early stages of a Watergate," the leader of Britain's Social Democratic Party, David Owen, today called on Conservative Prime Minister Margaret Thatcher to "tell the truth" about the controversial torpedoing of the Argentine cruiser Belgrano during the 1982 war over the Falkland Islands.

In a blistering attack on the government's handling of lingering questions about the sinking, Owen said the government had "locked itself into a depressing cycle of error, half-truths and deliberate falsehoods."

Owen, a former foreign minister under the ruling Labor government in the late 1970s who left that party to help found the more middle-of-the-road Social Democrats in 1981, is one of the most respected spokesmen in Britain on foreign affairs.

With his speech today at the party's annual conference at Buxton, he became the first leader of any major party to publicly lend his support to demands for full disclosure to Parliament and the public about the sinking of the Belgrano which took the lives of 368 sailors.

Owen is on the conservative side of his own party and has sided with Thatcher on some key issues, such as not giving in to Britain's striking coal miners. Until today, he had, in fact, come under some criticism at his party conference for too often appearing to mirror Thatcher's policies.

The circumstances surrounding the sinking of the ship on May 2, 1982, by the British nuclear-powered submarine Conqueror have become the subject of renewed controversy here in recent weeks after documents, allegedly from the Ministry of Defense, were leaked to a Labor member of Parliament, Tam Dalyell, and published in the magazine New Statesman.

Dalyell, who has carried on a one-man crusade questioning the sinking for the past two years, has long maintained that the cruiser was sunk to sabotage peace efforts by the government of Peru rather than because it was a threat to the British task force that was heading for the islands, as the government maintains.

Dalyell and the magazine say the documents, the authenticity of which has not yet been challenged, show that the cruiser had changed course and was heading away from the British task force 11 hours before it was torpedoed, that the rules of engagement had been changed by the British without notifying the Argentines, and that two members of Thatcher's war cabinet had reservations about plans to sink an Argentine aircraft carrier before the Belgrano was actually attacked.

On Sunday, The Observer newspaper also said evidence it had obtained showed why former defense secretary John Nott gave false statements to the House of Commons two days after the sinking and why ministers have "since tried to conceal the truth from Parliament and the public for more than two years."

The newspaper noted that both Nott and Thatcher had told an alarmed Parliament on May 4 that the cruiser had been spotted at 8 p.m. on May 2 "closing on the task force" and that sinking it was "entirely consistent with our inherent right of self-defense under the U.N. Charter."

The newspaper reported that on the morning of May 2, intelligence had been received in London that the cruiser was closing on the task force and that a Downing Street decision to attack it was made around 1 p.m., with the order transmitted an hour later. But at about that same time, the paper said, the submarine was reporting new intelligence that the cruiser had reversed course.

The question, the paper said, was whether the war cabinet was aware of this new intelligence at the time. The actual attack was said to have been carried out several hours later, at around 8 p.m.

The Observer quotes an unidentified government source as saying that "Thatcher and Nott seemed frightened to admit what they had done at the time, and they were stuck with their statements afterward."

So far, the government has said nothing about the new allegations. This has irritated some of its supporters, who argue that the military case for sinking the Belgrano was a valid one that can be supported, that there is no evidence it was done to sabotage a peace plan and that the government is hurting itself unnecessarily by failure to make details public.

The Economist magazine last month had asked the government in a headline: "Why not come clean?" on the Belgrano case. It argued that the government's passion for secrecy had caused it to tie itself into knots and that the military balance confronting the British fleet as it sailed near the islands was dangerously stacked against it.

The magazine said Argentine sources have since admitted that they planned the encircling maneuvers of the approaching fleet that British admirals feared, that the day before the Belgrano was sunk Argentine planes had attacked a British vessel, that the opportunity to eliminate major ships opposing the British force could not be missed and that the decision to sink the cruiser "was hardly a unilateral escalation of the war."

Owen said today that he had not before either criticized or endorsed the sinking and had no intention of doing so now. But information has emerged, he said, that the government had not told the full truth to Parliament or the public and "the crux is that the record must be set straight."

Only then, he said, "will we avoid a long . . . and bitter dispute which will only deepen the mistrust that people have of politicians and further weaken the good name of the British Parliament.

"The truth is not discreditable," he continued. "It simply needs to be told. But instead, we have a campaign of misinformation, beginning to reach into the heart of democratic government. The integrity of the civil service is being brought into question. We are in the early stages of a Watergate and unless the government, and in particular the prime minister, Mrs. Thatcher, steps forward and promptly tells the plain, unvarnished truth to the British people, the situation will get worse and worse."

Owen called for dropping the prosecution under Britain's Official Secrets Act against a Defense Ministry official, Clive Pointing, who allegedly leaked the documents to Dalyell. Pointing is a member of the Social Democratic Party, and Owen said "we believe civil servants should not be placed by any government in a position where they are made a party to false information given to Parliament."