The government of North Yemen last night formally requested that Marxist South Yemen be expelled from the Arab league on the grounds that it is a tool of the Communist powers and was responsible for the assassination of North Yemen's president a week ago.

At an unusual public session of the league that dramatized the deep split in the Arab world between friends and foes of the Soviet Union, North Yemen's foreign minister, Abdullah Asnaj warned that "this will not be the last crime carried out by the Aden government" because, he said. South Yemen is an agent of the strategic ambitions of one of the super powers.

It was clear that he meant the Soviet Union and, in a sense, it was actually the Soviets whom he sought to put trial as the delgates to an emergency league meeting went into closed session to consider his appeal.

The Arab states that are most closely aligned with Moscow, including South Yemen itself, boycotted the meetings, as they have all Arab League functions in Cairo since the visit to Israel of Egyptian President Anwar Sadat last year.

All members of the so-called stead-fastness front formed to oppose Sadat's peace initiative - Iraq. Libya, Syria, Algeria and the Palestinians, as well as South Yemen - stayed away. Djibouti, a small African state separate from the two Yemens by the narrow Bab El Mandeb Strait, was also absent.

Conservative Arab countries, including some like Egypt and Sudan that once had close ties to Moscow, have responded with alarm to the latest wave of violence in the two Yemens, which they see as fresh evidence of Soviet ambitions to dominated the Arab world and the Horn of Africa.

The countries are deeply anxious about the radical government of South Yemen, which is the only avowedly Marxist state in the Arab world and has closed ties to Cuba, East Germany and Ethopia as well as to the Soviet Union. Led by Saudi Arabia, the conservative and pro-Western Arab states have expended much money and effort in recent years in a vain attempt to loosen the Communist grip on Aden.

It appeared unlikely, however, that South Yemen would be expelled from the Arab League, since the league charter requires that such an action be aunanimous except for the accused member. No nation has been expelled since the league, which now has 22 members, was founded in 1945.

Last night's emergency meeting, at which the Yemen issue was the only item on the agenda, opened with a moment of silence in memory of North Yemen's slain president, Lt. Col. Ahmed Ghashmi.

Then the secretary general of the league, Mahmoud Riad of Egypt, called on Asnaj to make his presentation. He did so in full view of television cameras, to the surprise of observers who had expected the entire session to be held behind closed doors.

The participants did go into closed session ot consider North Yemen's charges against its neighbor. No decision is expected before today.

The presidents of North and South Yemen were both slain within 48 hours of each other last month in the latest eruption of violence in Yemeni politics, a murkey but votatile brew of ideology and tribalism.

Ghashmi, according to official North Yemini statements, died when a South Yemeni official who came to see him at his office in Sanaa opened his briefcase and it exploded.

Ghashmi had come to power only last October, after the assassination of his predecessor, The Sanaa government immediately accused the South Yemenis, then nominally under President Salim Robaya Ali, of being responsible for Ghashmi's murder.

South Yemen stoutly denied it, but two days later Robaya Ali was himself executed by former colleagues following a leftist palace coup.

The prevailing theory around the Arab world, is that Robaya Ali's associates in the ruling Popular Front seized on the assassination of Ghashmi to do wha they allegedly wanted to do anyway - get rid of Robaya Ali because he was less doctrinaire than they and more pragmatic in dealing with non-socialist states. he had opened diplomatic relations with Saudi Arabia and on the day of his death was scheduled to receive an official American delegation.

In his remarks last night to the Arab League, Foreign Minister Asnaj went further. He suggested that the new rulers in South Yemen themselves plotted the killing of Ghashmi to create the pretext for ousting Robaya Ali. He said the crime was "an extension of the power struggle" in Aden, itself" a tool of the strategic goals of one of the big powers."

Asnaj, himself of South Yemeni origin and a former leader of that country's struggle for independence from Britain, said it had been proved that the carrier of the briefcase bomb was seen off at Aden airport by an official of the Aden government, and had gone directly from Sanaa airport to his meeing with Ghashmi - evidence, he said, that "this crime was planned in Aden."

He spoke briefly, in calm tones that belied the agitation last month's murders cause throughout much of the Arab world. The pro-Western Arab states around the Red Sea - Saudi Arabia, Sudan and Egypt - appear particularly concerned by events that amount to a policy setback for them.

Asnaj met yesterday with Sadat who, he said, told him the events in Yemen were "the beginning of a plot to involve all Arab countries."

Whatever the outcome of this Arab League session, the latest developments seem certain to put an end to talk of merger of the two Yemens or at least of closer cooperation between the two, and to dash the hopes of South Yemen's monarchical neighbors,Oman and Saudi Arabia, for a period of stability at the tip of the Arabian peninsula.