Hard-line Marxist opponents of South Yemen's president said in a radio broadcast yesterday that they have taken control of the war-torn country on the southern tip of the Arabian Peninsula and intend to "tighten its strategic ties with socialist countries, especially its ally, the Soviet Union."

The whereabouts of President Ali Nasser Mohammed, a Marxist considered too pro-West by his rivals, was unknown. Addis Ababa radio said that after flying to Ethiopia on Saturday for consultations with leaders of that country, Mohammed left yesterday for "home," but there was no confirmation. Other reports placed him in North Yemen and in Moscow.

Amid reports of continued fighting and confusion in the Arab world's only Marxist state, an international rescue effort led by the British royal yacht Britannia and several Soviet vessels resumed. Several hundred more foreigners were evacuated from the beaches of Aden, the capital of South Yemen, to join the nearly 3,000 already in Djibouti, on the Horn of Africa, a seven-hour voyage away.

The British Embassy here said yesterday that those evacuated Friday included an American, identified only as Hazel Denton, reportedly a resident of Washington. British officials said Denton had gone to the U.S. Embassy in Djibouti, but a State Department spokesman said yesterday that he had no information about her. South Yemen broke diplomatic relations with the United States in 1969, and the State Department has said repeatedly since the civil war broke out last Monday that it had no reports of Americans in the country.

A radio station that said it was transmitting from Aden on behalf of an unidentified new leadership broadcast a communique yesterday indicating that the rebels had overthrown Mohammed, the Bahrain-based Gulf News Agency reported, according to The Associated Press.

The broadcast said that South Yemen's 15-man Politburo had deposed Mohammed and accused him of irresponsibility, dictatorship and attempting to "physically liquidate" them. The new leadership, it said, had received the support of the armed forces and had ordered the military to halt the fighting that it said had killed or wounded 9,000 persons.

Monitoring services said, however, that while the broadcast used the frequency of the official Aden radio, it could not be determined whether it was actually coming from the capital or from elsewhere in the country.

With international communications cut off, and most western embassies evacuated, it was impossible yesterday to confirm the various accounts of the situation there.

The official Kuwaiti news agency, quoting sources in North Yemen, said the battlefront in South Yemen was "quiet" but remained "confused," United Press International reported.

Radio Moscow reported that "armed clashes are in progress in various regions of Aden," and it added: "A great deal of damage has been done. There are considerable human casualties. Efforts to put an end to the bloodshed are continuing."

In Djibouti, many of those evacuated told of heavy fighting in Aden, Reuter reported. Some said they saw many dead and wounded in streets near the main luxury hotel, the Frantel.

Associated Press correspondent Alex Efty, reporting from the Quraish border post on the main highway linking North and South Yemen, said a Palestine Liberation Force convoy of 30 vehicles and 300 troops attempting to go to Aden as a peace-keeping force had been barred from entering the country and spent the day parked on the roadside.

Guards on South Yemen's side of the border told Efty that fighting "is still continuing" in Aden and that no traffic had been allowed through since Monday.

The civil war broke out Monday, reportedly after an attempt by hard-line Marxists to assassinate Mohammed, who, although a staunch Soviet ally, had angered them by his efforts to improve economic relations with conservative Arab states in the region and with Western Europe.

Aden radio reported then that the four alleged ringleaders of the coup attempt, including former president Abdel Fatah Ismail and Vice President Ali Ahmed Nasser Antar, had been executed, but unconfirmed reports since then have said they were alive and heading the rebellion.

Mohammed, according to Ethiopia's state radio, left Addis Ababa yesterday after conferring with that country's Marxist leader, Mengistu Haile Mariam. Although Ethiopian radio said Mohammed had gone "home," Mickey Gurdus, an Israeli who monitors airplane transmissions, said that a Yemeni plane that said it was carrying an unidentified "senior" official flew from Addis Ababa to Taizz, North Yemen.

Cairo's Al Ahram newspaper reported in today's early editions that Mohammed flew to Moscow with five aides, but there was no confirmation from Moscow. South Yemeni Prime Minister Haidar Abu Bakr Attas, who was in New Delhi when the rebellion began, flew to Moscow last week for consultations and remained there yesterday, issuing a new appeal over Radio Moscow for peace in his country.

South Yemen is the Soviet Union's closest ally in the Arab world and has allowed Soviet ships and planes to use facilities in Aden and Socotra island. More than 2,000 Soviet advisers and technicians and their families have been evacuated from Aden during the past two days.

British Ambassador Arthur Marshall, who went to Djibouti on the Britannia Saturday but returned yesterday "to look after the rest of his flock," praised the Soviets for making the evacuation possible by "negotiating a kind of cease-fire for these unruly tribes and militia . . . after a hellish five days."

British officials said there were about 1,000 foreigners still in South Yemen seeking to leave.