The siege of the U.S. Embassy in Tehran has reversed one of the few cardinal rules of dealing with such mass kidnapings. In Iran, time is on the side of the captors and works against those who are trying to free the hostages.

Because each passing and torment-filled day adds to the domestic and international political advantages sought by the Iranian extremists who hold the embassy, their promise not to kill their hostages can probably be taken seriously. The status quo is ideal for the extremists' aims.

That is true in part because their ultimate target appears to be not the hostages, nor even the return of the ailing shah. With Ayatollah Khomeini's connivance, the Tehran mob is challenging the U.S. position in the Third World and, in a sense, America's national spirit.

The televised images of truckloads of cheering Iranian workers and farmers hooting past the captured embassy this weekend, screaming support for the captors, will have made that point graphically for may American viewers.

Ironically, many of those viewers switched to the Tehran news film from Sunday night's broadcast of "Dog Day Afternoon," the Al Pacino movie about a hostage-taking in a Brooklyn bank. In the film, the police followed standard tactics and stretched out negotiations as long as possible to wear down the captors and prevent them from harming the hostages in panic.

President Carter and Secretary of State Cyrus R. Vance endorsed such tactics earlier this year when the U.S. ambassador to Afghanistan. Adolph Dubbs, was held hostage by terrorists in Kabul. Ignoring pleas from Vance to continue negotiating with the terrorists, Afghan police and their Soviet advisors rushed the terrorists and Dubbs was killed in the shooting.

In Tehran, Carter confronts perhaps an even more painful and difficult dilemma. The option of dragging the siege out lies with the captors. They watch with evident glee as each day brings a rise in the tarnishing of American prestige in the world and in national frustration here.

A U.S. military strike into the center of crowded Tehran might staunch this hemorrhaging of pride. It would also probably result in the death of the 60 to 65 Americans thought to be in the embassy. It is a trade-off that Carter thus far has not been willing to make.

The course of the siege has made it clear that this is terror with a difference, and that Carter has to choose a different approach. The goals and tactics of the Iranian militants are quite different than those of the Black September group that kidnaped U.S. Ambassador Cleo Noel and his deputy, G. Curtis Moore, in Khartoum, the Sudan, in 1973.

President Nixon decided immediately to refuse to negotiate with the Arab terrorists over the fate of the two men. On hearing this, the gunmen murdered Noel and Moore and a Belgian diplomat they had also seized.

Carter has refused to oust the shah, who is recovering from cancer surgery in New York. In addition to dispatching special negotitators, the president has asked other nations to intervene with the 79-year-old ayatollah, but with no success.

In this siege, it is Carter who must race the clock. His most immediate task appears to be trying to get slightly in front of and channeling the building U.S. anger. He is taking steps that do establish American ability to act. His order yesterday to cut off Iranian oil imports will have more symbolic than practical impact.

The clock has been turned on its head in this case because the siege of the embassy is a key weapon in a power struggle inside Iran. Islamic extremists used the takeover to sweep away the weak but Western-oriented government of Prime Minister Mehdi Bazargan. Each day of demonstrations around the embassy consolidates the radicals' hold on power.

Moreover, the militants have pinned the Carter administration once again to the legacy of a quarter century of supporting the shah, a legacy the State Department had hoped to forget.

Despite the terror tactics being used, the new demand for an American admission of national guilt as part of the price of freeing the hostages will be popular in parts of the Third World that feel the United States has backed too many "dictators" in too many places in the Cold War era.

Carter and his principal advisers came to office declaring a fresh start for American relations with the Third World. They pledged to eradicate the covert operations and policies that had often inflamed world opinion against America abroad.

Their record has been mixed. Carter did attempt to move away from Chile-type operations. But that movement could well end through the Pyrrhic "victory" the ayatollah's men think they are achieving.

The siege is resulting in a daily coarsening of the American spirit, the ultimate hostage in this siege. The longer it continues, the less chance there is for Carter or his successor to find significant support for a sympathetic policy toward the Third World. Calls for a return to convert intervention of the kind that brought the shah back to power in 1953, and even more belligerent action to reestablish American prestige point to quite a different global future than the one Carter seemed to envision on election day three years ago.