Britain's royal yacht and four Soviet ships evacuated 1,700 foreigners from South Yemen yesterday, as opposing forces fought fiercely for control of the Marxist-ruled country and unconfirmed reports said its president fled to Ethiopia.

Refugees and British officials told of emotional scenes and a harrowing rescue from the beach at Aden, South Yemen's capital, as rebel forces advanced toward it and families waded through neck-deep water to the safety of Queen Elizabeth II's royal yacht Britannia.

"I can tell you, when they sighted the royal yacht all lit up, there were lumps in everybody's throat," British Ambassador Arthur Marshall, one of the 400 refugees evacuated by the Britannia, told British journalists by radio, Washington Post correspondent Karen DeYoung reported from London.

"What we would have done with this battle about to burst out if the Britannia hadn't been there, I just shudder to think," Marshall said. "It would have been a bloodbath of unthinkable proportions."

Diplomats in the region said fighting intensified yesterday, The Associated Press reported from Bahrain, and the Bahrain-based Gulf News Agency, citing official sources in North Yemen, said as many as 9,000 people had been killed or wounded in the fighting in South Yemen.

The British and Soviet ships took the refugees, who reportedly included 1,000 Soviets and most of the last westerners in the staunchly pro-Soviet country on the southern tip of the Arabian Peninsula, across the Bab al Mandab strait to Djibouti. From there, Soviet and other airliners were preparing to fly them home.

The civil war in South Yemen, site of important Soviet air and naval bases, erupted last Monday, apparently with an attempt by Marxist hard-liners to assassinate President Ali Nasser Mohammed, whom they considered too friendly with prowestern Arab neighbors. It reportedly has divided the country's armed forces, which are almost completely Soviet-armed and trained.

Aden radio reported last week that four leaders of the rebellion -- including former president Abdul Fatah Ismail and Vice President Ali Ahmed Nasser Antar -- had been executed. Evacuees arriving in Djibouti yesterday said, however, that since then Aden television had reported that Ismail and Antar were alive, Reuter reported.

Israeli state radio, quoting Michael Gurdus, an Israeli who monitors airline radio frequencies, said that an official South Yemeni plane reportedly carrying Mohammed flew to North Yemen and then to Ethiopia yesterday, but there was no other confirmation.

Ethiopia originally refused the South Yemeni jet permission to land but granted permission when the pilot confirmed the president was aboard, Israeli radio said, according to United Press International.

North Yemen, which has sporadically been involved in hostilities with South Yemen, did not mention Mohammed, but its president, Ali Abdallah Saleh, called last night for a cease-fire in Aden and for representatives from both sides to come to Sanaa, the North Yemeni capital, to resolve their differences, Agence France-Presse reported.

The Gulf News Agency said Palestine Liberation Organization Chairman Yasser Arafat arrived in Sanaa yesterday and met with Saleh to discuss efforts toward a truce in Aden, where several PLO factions opposed to Arafat's leadership have offices.

From Moscow, South Yemeni Prime Minister Haidar Abu Bakr Attas and Foreign Minister Abdul-Aziz Dali issued an appeal broadcast over the Arabic-language service of Moscow radio for an end to the fighting. Both had been in New Delhi when the civil war broke out and went to Moscow to consult and to await developments.

The Britannia, a 412-foot vessel built as a hospital ship, had been en route to New Zealand for use by the queen during a February visit to the region when it was diverted to Aden to assist in the evacuation. It has a crew of 276 and room for 700 passengers.

Its captain, Adm. John Garnier, said by radio that he was forced to leave some people in Aden when heavy fighting made it too dangerous to continue the rescue mission. Other British vessels remained in the area, however, and officials said they were awaiting opportunities to remove those who want to leave.

During the evacuation, Garnier said, "heavy shelling started at Khormaksar airport and started to creep toward the embarkation area. Shortly after that we saw, about a mile-and-a-half up the beach to the north, tanks and rebel militia advancing down the beach.

"By the time we had withdrawn the shore party . . . the rebels were about a mile away, and I then went out to sea to transfer some of the French nationals to the French ship. We went back to the beach, and there was a very considerable battle going on, actually where our beach party had been. With the heavy fire fight and the high caliber shells falling on the buildings just at the back of the beach, it certainly wouldn't have been tenable" to try to go back ashore.

Daryl Barker, a British resident in Aden, said in a ship-radio interview with British journalists that he and his wife, with their 16-month-old daughter, "had to wade into the water waist high, and then some of the waves were higher -- up to the neck. I managed to get my wife on one boat, and one sailor had Francesca presumably his daughter and took her in another boat."

Garnier said there were three British citizens known still to be in the Aden area, "pinned down in an air raid shelter," and about 45 others in the countryside and not thought to be in immediate danger.

State Department officials have said they have no reports of U.S. citizens in South Yemen, which broke relations with the United States in 1969.