Seven White Roman Catholic missionaries were shot to death by a dozen armed black men at the Masumi mission, about 57 miles cast of here, last night. The sole survivor of the massacre said he was sure the killers were nationalist guerrillas.

Rhodesia's black nationalist Patriotic Front, however accused the government of Prime Minister Ian Smith of the killings, saying that the missionaries "stay among the people" and "see every day the atrocities that the racists carry out on the struggling masses."

The Rev. Dunstan Myerscough, 65, sole survivor of the attack, said gunmen ignored black nuns and staff members and "did not say one word about why they were doing this to us."

"When one of the sisters asked what they wanted," Father Myerscough said, "one of them said, 'We want our country.'"

The slain missionaries included three Jesuits - two British priests and an Irish brother - and four Dominican nuns, three from West Germany and one from England.

Father Myerscough, a Jesuit from England, said that shortly before 10 p.m. a nun knocked on his door and asked him to come out.

"I opened the door and walked into the wrong end of a gun," he said. One of the gunmen took his watch and, later, his glasses, and herded the group out into the mission compound.

Then, the priest said, the gunmen appeared to argue as to who would do the shooting. "As soon as I saw the burst of fire," he added, "I sort of - I don't know whether instinctively or imagining I was hit or what - I fell flat down." When he heard the gunmen running away he discovered he had not been shot.

"I got up and realized there was nothing I could do for any of the others, and I went back into the house" to telephoned for help.

In the mission and elderly German nun, identified only as Sister Anna, said she had been spared because her arthritis kept her from moving fast enough.

"There was a knock on the door and as I opened a terrorist kept saying, 'Get out! Get out!," she said.

"I slipped and fell to the floor," she added. "He kept telling me to 'Get up! Get up!' I told him to give me a chance. I have a sore leg and I'm not young."

He saw my watch, took it and went to another room" to round up others, she said.

Rhodesian police said later that they had collected 111 spent cartridges from Soviet-made machine guns at the mission, which comprises primary and secondary schools, a social center, hospital and farm.

Besides the missionaries, there were some 400 boarding students and about 150 African staff members, including some nuns, at the mission at the time of the shooting. The schools also have about 300 day students.

The Catholic Church, which has some 600,00 members in Rhodesia and nuns more missions here than any other church, has been sharply critical of the country's white-minority government and its racial policies.

Today, Salisbury's black Catholic archbishop, the Most Rev. Patrick Chakiapa, called the attack an "evil act" that makes a "mockery of whatever good ideals [the gunmen] claim to serve."

Pope Paul, in a telegram to Archbishop Chakiapa, said he was "profoundly grieved" and denounced the killings as an "atrocious" and " detestable act of violence."

Last night's attack is by far the most serious against any church institution in Rhodesia and the largest massacre of whites in the four-year-old civil war here.

In september, Catholic Bishop Donal Lamont of Umtali, Rhodesia, was sentenced to 10 years in prison for allowing guerrillas to get medical treatment at a mission. He is appealing the sentence.

Meanwhile, in Bulawayo a Catholic priest sait that more than 300 youths who chose to remain in Botswano despite their parents' pleas had been "brainwashed and fed on a diet of threats and promises."

The priest, the Rev. Edgar Sommerreisser, accompanied 140 parents across the border yesterday in a generally vain effort to get the youths to return to Manama from Botswana, where they had been taken by guerrillas. Only 32 chose to return, most of them girls between 13 and 16.