Margaret Thatcher, the Conservative Party's determined crusader against socialism, will become the first woman prime minister in Britain and in Europe following a comfortable but not overwhelming Conservative victory in Thursday's British election.

After more than three-fourths of the 635 seats in the House of Commons had been decided by 5:30 a.m. (12:30 a.m. EDT) in the slow-moving count today, news organizations here estimated by computer that the Conservatives would win about a 30-seat majority in Parliament.

With 496 constituencies counted-and many safe rural Conservative constituencies not set to be counted until later today-the Conservatives had won 249 seats to 237 for the Labor Party, 6 for the Liberal Party, 2 for the Scottish Nationalist Party and 2 for the Welsh nationalist party Plaid Cymru.

Overall, the Conservatives were winning nearly 43 percent of the vote, Labor 40 percent and Liberals 13 percent. The Conservatives increased their vote by 8 percent since the last election, in 1974, while Labor's vote fell sharply everywhere but in Scotland. The Scottish Nationalist Party suffered a severe setback in Scotland, losing at least four of its eleven seats in Parliament to Labor or Conservative challengers.

Although the middle-ground Liberals lost a significant portion of their 1974 vote nationally, they had lost only one of their 14 seats in Parliament by early this morning, former Liberal leader Jeremy Thorpe's seat un rural North Devon. Thorpe, who is to stand trial here next week on charges of conspiring to murder former male model Norman Scott, lost by a wide margin to a Conservative challenger after representing North Devon in Parliament for 20 years.

Neither Thatcher nor outgoing Labor Prime Minister James Callaghan would claim victory or concede defeat this morning, preferring to wait until the results are known in the more than 100, mostly rural constituencies where counting will not begin until later today.

"I am still not absolutely certain," Thatcher told newsmen at Conservative headquarters in London at 4 a.m. "We were cautiously optimistic. Now we are optimistic."

Asked how she felt, she said, "I'm just aware of the very great responsibility, but it is exciting. But somehow one is very calm about it-it needs that calm. But the people who have helped me through all these days are still here, and they are still helping me."

Callaghan, speaking a few minutes later from the Cardiff, Wales, constituency from which he won re-election to Parliament, said, "I've been watching the results with very great interest. I've seen the trends. I shall have something more to say [later today] in London after it's entirely clear."

The regional differences in the results were sharper than in the recent past, with Labor increasing its grip on Scotland and northern England, while the Conservatives made their expected large gains in southern and central England. Conservatives increased their strength in suburban and rural areas, while Labor remained in control of most central cities.

Thatcher's new Conservative government thus will have a modest majority in Parliament with which to try to make major changes in a nation rather divided-Conservative south vs. Labor north, Conservative suburbs and countryside vs. Labor cities.

Thatcher, 53, took over the party and moved it to the right after the Conservatives lost two successive elections to the Labor Party in 1974. She has vowed to revitalize Britain by cutting income taxes, reducing government involvement in the economy and curbing the power of labor unions.

Thatcher will be the first woman to head a parliamentary government in any country west of Israel. Golda Meir in Israel, Indira Gandhi in India and Sirimavo Ratwatte Dias Bandaranaike in Sri Lanka preceded her as democratically elected female prime ministers elsewhere in the world in recent years.

Callaghan has defended the outgoing Labor government's expansion of Britain's welfare state as having improved the lives of working people and the poor. Labor, which won four of the last five national elections here, has run Britain's government for 12 of the last 15 years.

Callaghan, 67, who became prime minister when Harold Wilson suddenly retired three years ago, again said he would not step down as leader of the Labor Party no matter what happended in the election.

"Win or lose," he siad, "I intend to lead the party for a full parliamentary term of five years. There is no question of my retiring."

Callaghan dismissed as "Conservative scares" speculation during the campaign that he would retire within two or three years and that the Labor Party's leadership might go to someone in its growing left wing.

"I do not accept it is likely that I am going to be pushed out," he said.

The tall, silver-haired Labor Party leader, who wears thick, dark-rimmed glasses and hides a strong temper behind an easy smile, is Britain's first prime minister in this century to have held each of the three top Cabinet positions in government-foreign secretary, home secretary and chancellor of the exchequer.

In a stately, low-key campaign of chats and speeches to the party faithful, Callaghan presented himself as a steady, experienced statesman, a veteran of countless summits with world leaders and a calm, centrist fine-tuner of the complicated government machinery in the welfare state.

The contrast with the less experienced, stridently aggressive Thatcher, who lectured Britain like a stern schoolmistress during her slickly packaged, media-oriented campaign, boosted Callaghan's personal popularity to double hers in opinion polls.

But there was also another strong theme in the pollster' findings: voters want a change. They are wary of Thatcher, but they strongly support her proposals to cut taxes, to reduce government spending and power, to curb the labor unions, and to strengthen the forces of military defense and domestic law and order.

They are frustrated with rising prices and tax bills. They are still angry about the long strikes that exacerbated the difficulties of a long, cold winter and shut schools, curtailed the operations of hospitals, left garbage uncollected, cut off some supplies to businesses and stores, and even delayed the burying of dead in some parts of the country.

They feel beset by crime and vandalism, despite statistics showing the national crime rate is falling, and they are suspicious of the growing minority population of black and Asian Commonwealth immigrants in many urban areas.

In an interview, Thacher said, "What we offer is lower taxation because we believe people are entitled to a bigger proportion of the fruits of their own efforts."

Thatcher, whose only Cabinet experience was a education secretary, capitalized on this uneasy national mood with an evangelical crusade against socialism and for private enterprise "to restore freedom" and "make Britain great again."

In appealing to undecided voters who want change but might vote for a third party, Thatcher said at her last campaign press conference, "The only way to fight state socialism as we've got it at the moment is to fight it head on and beat it head on."

Thatcher said, "What we offer is lower taxation because we believe people are entitled to a bigger proportion of the fruits of their own efforts."

"If they don't get that," she told Britain's Press Association, "they won't work harder. And if they don't work harder, we shall not get expansion. We shall not get a higher standard of living or the money for helping the disabled or improving the prospects for our children."

Built around this philosophy are Conservative proposals to regulate the economy through control of the money supply rather than government review of wages and prices, to make management and labor more equal in negotiations through legislation limiting union power, and to reduce the government's role in the economy by selling some of what the government owns, from the aerospace industry to public housing.

In response, Callaghan warned that the Conservatives' income tax cuts would have to be paid for with large increases in sales taxes and that big cuts in government spending would put employes of nationalized industries out of work and cripple social welfare programs. He also argued that a confrontation with labor unions would lead to strikes much worse than those of last winter.

The young leader of Britain's Liberal Party, David Steel, offered the Liberals as a progressive alternative to the "extremism" of Labor on the left and the Conservatives on the right. Steel, 40, hoped that neither major party would win a majority of the seats in Parliament so that one of them would have to form a coalition with the Liberals to govern.

The ultimate strength off the Liberals and the nationalist parties in Scotland and Wales in the next Parliament will not be known until late Friday. Ballot counting in more than 100 of the 635 parliamentary contests, some in previous minority party strongholds, has not yet begun.

Also among these are the 12 constituencies in Northern Ireland, contested only by a bewildering variety of Protestant and Catholic sectarian parties, plus one party, Alliance, that is trying to bridge the sectarian gap. Most of the seats are expected to be Protestant Ulster Unionist factions. CAPTION: Picture, Conservative leader Margaret Thatcher leaves her voting station in London. UPI