With language that recalled the rhetoric of the Cold War, the General Assembly opened the fifth emergency session in its 34-year history today with a debate on the Soviet invasion of Afghanistan.

Although the Soviet Ambassador here was slated to speak in defense of his government's Afghan action, he gave up the opportunity at the last minute. Instead, Eastern bloc member Poland took the initiative to "strongly object" to a meeting called by "reactionary elements" as a "smoke screen to cover [their] agressive designs."

Japan.Senegal, Colombia and China spoke in favor of a resolution, vetoed by the Soviets in the Security Council last night, calling for troop withdrawal.

Chinese Ambassador to the United Nations Chen Chu said the Soviet invasion was part of a "global strategy" intent on "dominating the world." Chen compared the Soviet move to Hilter's invasion of Austria and Czechoslovakia at the start of World War II.

On the question of Iran, The United States today picked up a crucial ninth Security Council vote, reportedly from Tunisia, for the imposition of economic sanctions.

Diplomatic sources said the U.S. delegation is expected to submit its sanctions resolution to the council Friday and ask that it be tabled. Under U.N. rules, a tabled resolution must to be taken by the Security Council within 24 hours, and a council meeting presumably will be held by Saturday.

The tabling move is a procedural one, designed to take the initiative away from the Soviets. If the United States were to introduce the resolution tonight and call for an immediate debate, as was expected earlier, any other member could slow its momentum by calling for a 24-hour waiting period.

The United States, sources said, expects a Soviet veto. The resolutions calls for the prohibition of all sales and supplies, except food and medicine, to Iran by U.N. members until the estimated 50 Americans held hostage in Tehran are released.

Today's emergency special session was the first since 1967, when the Soviet Union requested a meeting on the Middle East.

In preliminary discussions before the meeting officially was opened, several Sovet bloc members objected to holding it at all. They said that the Afghan situation did not meet the prerequisite of threatening international peace necessary for such as extraordinary session.

Once the session was opened, new Afghan Foregin Minister Shah Mohammad Dost repeated those arguments, plus points he and the Soviet Union had made in the previous Security Council debate. Dost said the Soviet Union's "limited mission" in Afghanistan was by navigation and was necessary to "repel foreign aggression and armed intervention" from other unnamed countries.

During the debate, it was alternately argued that each of the superpowers was engaged in efforts to take over the world.

Dost reiterated the Afghan-Soviet position that Moscow "had nothing to do whatsoever with events of Dec. 27." On that date, former Afghan president Hafizullah Amin was assassinated and Babrak Karmal was installed as chief of a new government. He said that current Soviet Military assistance is a bilateral matter between the two countries.

Japanese Ambassador Masahiro Nisibori countered that the "international Community remains clearly unconvinced of the Soviet explanation" for the presence of troops that the West and much of the Third World charge invaded Afghanistan on Dec. 24, ousting Amin and now occupying the country.

The most colorful language used in today's debate, which is scheduled to continue Friday and should last several days, was provided by Chinese Ambassador Chen.

In answer to what he said were Soviet assertions that its troop presence came in response to "an invitation of Mr. Amin," Chen said, "Obviously, Mr. Amin was not the stupid."

Chen asked rhetorically if Babrak had issued the invitation, but then said, "He was not at that time the head of the Afghan government. It was the Soviet government itself who invited Soviet troops into Afghanistan."

The Soviet Union's sponorship of a U.N. resolution last year condemning expansionism "is like a thief crying 'stop thief," Chen said.

Once the debate is over, the assembly likely to pass, by the necessary two-thirds majority, a resolution condemning the Soviets that is similar to one voted by the Soviets in the Security Council. Since an assembly resolution calling for Soviet troop withdrawal does not carry the weight of a Security Council resolution, its supporters agree any victory will be a purely moral one.

a similar situation may yet occur on the Iran question. The United States reportedly is counting on the council votes of France, Britian, Norway, Portugal, the Phillippines, Niger, Jamaica, Tunisia and itself to make up the necessary nine. Mexico and China in the past several days have shown reluctance over the sanctions question, and their positions are unclear.. s

Assuming a Soviet veto of the sanctions resolution, however, the United States will have three remaining options: it can do nothing about santions; it could institute unilaterial restrictions outside the United Nations, or it could, under the same special veto-less provision used on the Afghan question, bring the issue to the speical assembly.

An assembly resolution on sanctions -- which, in an case, is considered unlikely to win the votes of two-thirds of the United Nations' 152 members, would have only moral, rather than legal authority.