Tens of thousands of demonstrators formed a spectacular human chain that snaked through 14 miles of English countryside today, kicking off the year's major protests against deployment of cruise missiles in Britain.

The chain stretched from the barbed wire of Greenham Common, a U.S. military base 50 miles west of London where the first cruise missiles are expected to be installed in December, to Burghfield, where Trident nuclear warheads will be manufactured.

The chain was incomplete at one point a few miles from Burghfield but in other places the demonstrators were more than five deep. The Campaign for Nuclear Disarmament estimated that 70,000 to 100,000 men, women and children participated--a turnout that would make it one of Britain's largest antinuclear protests. Local police put the figure at 40,000.

Demonstrations also were held in West Germany, Switzerland and the Netherlands as much of Western Europe prepared for an Easter weekend of protests against nuclear weapons. An estimated 10,000 demonstrators blocked entrances to six U.S. military bases in West Germany, and police used tear gas to disperse 250 protesters blocking the entrance to a base at Neu Ulm, The Associated Press reported.

The human chain was the climax of Britain's traditional Easter week peace demonstration and gave an inkling of what may lie ahead this year. Disarmament organizers say they will intensify their efforts this year because of the expected arrival of the first cruise missiles and because 1983 probably will be an election year.

Prime Minister Margaret Thatcher, who is committed to deploying the missiles, also seized the occasion, arguing that to assure peace, Britain must install the weapons as a deterrent against Soviet attack.

"It would make far more sense," Thatcher said in Parliament yesterday, "for those women to go and link hands around the Berlin Wall. If by doing so they manage to persuade the Soviets to take it down, to remove the guns, the dogs and the mines there to kill those who attempt to escape, they would be doing something." She referred to the women who have camped outside Greenham Common for months.

To drive Thatcher's point into the homes of television viewers, Defense Secretary Michael Heseltine flew to Berlin to inveigh against the Greenham Common demonstrators from the backdrop of the Berlin Wall.

The demonstrators dismissed as window dressing President Reagan's latest offer to the Soviets of a common limit in nuclear warheads of intermediate-range missiles.

Peter Foster, a banker from Brighton who braved the blustery weather for his first disarmament demonstration, said, "Reagan's a Californian, and by and large Californians aren't very sophisticated."

Because the Conservative Party is strongly committed to deploying the missiles and is in a good position to win the next general election, even many demonstrators acknowledge that they probably will not be able to prevent the installation of the subsonic cruise missiles.

"If Mrs. Thatcher is elected with a solid majority it will be difficult," Joan Ruddock, chairman of the Campaign for Nuclear Disarmament, said in demonstration headquarters. "Then we would have to take more nonviolent direct action."

Public opinion polls indicate that a majority opposes introduction of cruise missiles, although only about one-quarter go as far as Ruddock's campaign and urge that nuclear weapons already in place be dismantled.

The Labor Party this week committed itself, if elected, to a non-nuclear defense policy. Those in favor of disarmament hope to capitalize on the public opposition to deployment of the new missiles, but the Labor Party is torn by internal bickering and the Social Democrat-Liberal Party alliance has not displaced it. Moreover, polls indicate that the most pressing election issue will be the economy, not nuclear weapons.

The so-called "peace movement"--a term that the government rejects as a misnomer--also contains many factions and includes many who are nonpolitical and say they distrust all politicians. Some of the women who have been camping on the Greenham Common since September 1981 in highly visible protest say they will run for Parliament on their own, which could split the left-wing vote.

Greenham Common has become the symbol of what disarmament supporters call the "American occupation of Britain." Likewise, the women who live in tents near the entrance, and who occasionally clamber over the fence before being ejected by police, have become a symbol of a more spontaneous opposition to cruise missiles.

Although the camp at Greenham Common includes only women--they exclude men--more than a dozen other camps have sprung up at other bases or nuclear sites and most of these consist of both men and women. A poll in January found that one-third of those surveyed feel more favorable toward the campaign against cruise missiles as a result of the Greenham protests. Fourteen percent said they viewed it less favorably, and the majority said their views were unchanged.

"We'll give it a jolly good go" to keep the missiles out, said Thalia Campbell, one of the original 40 women to camp outside Greenham.