Zimbabwe-Rhodesia's continuing raid deep inside Mozambique cast a shadow today over next week's British-sponsored constitutional conference here designed to end the escalating guerrilla war.

A government communique, released in Salisbury as Prime Minister Abel Muzorewa left for the London conference, said at least 300 Patriotic Front guerrillas and Mozambican troops and 13 Rhodesians had been killed so far. It was thought that the toll could climb much higher.

British sources said the timing of the attack, just before the conference, gave cause for considerable concern. "Obviously it is not helpful," one source said. "You can draw your own conclusions."

Publicly, however, the British government and Robert Mugabe, leader of the guerrilla force under attack in Mozambique, took a low-key approach.

The British Foreign Office issued a noncommittal statement saying that a major purpose of British policy "is to stop the war. We are seeking a solution in which an internationally recognized Rhodesia can live at peace with her neighbors."

Mugabe, who arrived here today, condemned the raid but told reporters he did not think it would affect the conference starting Monday.

He attributed the raid to people in the Muzorewa government "panicking," and added that it reflected "some kind of insanity. They don't know what to do" to gain recognition and end the war.

Muzorewa defended the move, telling reporters at Salisbury airport: "We preempted a very dangerous development which, if it had been successful, would have been diastrous for my country." The military said the aim was "to destroy the buildup of integrated" Mozambican and guerrilla forces to prevent further incursions into Zimbabwe-Rhodesia.

At the same time the government said Zambian forces to the north had carried out joint shelling, with the guerrillas, of Zimbabwe-Rhodesia. This led to conjecture that similar attacks may be launched against Zambian forces.

The communique said the shelling helped rescue 10 guerrillas stranded on an island in the Zambezi River after Salisbury's forces sank their boat, killing eight.

Rhodesian forces have frequently attacked both countries, including Mozambican forces, but the government has always said in the past that its only targets were Patriotic Front guerrilla bases harbored by the two countries.

At the United Nations, Secretary General Kurt Waldheim said the attack on Mozambique "threatens the peace and security of the whole region," adding that it was bound to hinder the new peace efforts.

In Washington, a State Department spokesman said the attack against Mozambique was particularly regrettable just before the constitutional conference. "We would hope and still hope that the parties all would recognize the wisdom of restraint in this period so that the deliberations in London can take place in an atmosphere of cooperation and reconciliation," the spokesman said.

A Mozambique Defense Ministry spokesman said the attack was designed to damage the country's economy. He told of homes, schools, hospitals and bridges being destroyed and said Maputo's forces were inflicting "heavy losses" on the attackers who have penetrated up to 200 miles inside the border.

The Associated Press quoted a witness in Guija, 100 miles north of Maputo, as saying: "Rhodesian commandos fired their machine guns against civilians and threw grenades inside their houses."

The death toll of at least 300 was the first given by the Zimbabwe-Rhodesian military, raising the speculation that the final count in the continuing fighting would be much higher.

The highest single toll in the 7-year-old war occurred in a Rhodesian raid into Mozambique in November 1977, when more than 3,000 persons were killed. Initial Rhodesian reports had put that toll at 1,200 killed and the true scope of the casualties only became clear weeks later.

The Salisbury government disclosed today that the offensive, believed to be the largest it has ever mounted, is already the costliest of the war in terms of security forces losses. Salisbury said 13 soldiers were killed yesterday when their American-built Bell 205 helicopter was shot down. The original report yesterday said only one technician was killed.

All 13 were white, including two Australians, a Briton and a South African. The death toll in the war is rising steadily and may be as high as 100 a day. The vast majority of victims are black, often civilians.

Both the Zimbabwe-Rhodesian government and the guerrillas have said the war will go on despite the peace talks.

Mugabe said on arrival, "We can't just call a cease-fire because [British Foreign Secretary] Lord Carrington has called a conference. We weren't fighting for a conference. We're fighting for independence." He said they will stop fighting when independence "is coming."

Mugabe's companion guerrilla leader, Joshua Nkomo, is scheduled to arrive Saturday as is Muzorewa. Former prime minister Ian Smith, the last of the key figures in the talks, is due Sunday.

In another development, Mugabe told reporters that his organization is releasing four whites, including a missionary, and eight African nuns who had been taken across the border in recent months to Mozambique.

He justified the abduction of the religious workers from the Marymount mission in northeastern Zimbabwe-Rhodesia by saying government forces had planned to massacre them and blame the guerrillas. The government and the guerrillas have frequently accused each other of atrocities.