A. J. Cronin, 84, the author of "The Citadel" and "The Keys of the Kingdom" and other novels that delighted readers all over the world but induced only lukewarm receptions from critics, died Tuesday at a nursing home in Montreux, Switzerland.

His death was announced yesterday by members of his family, who said he died of acute bronchitis.

Mr. Cronin was a Scot by birth, a physician by training whose practice took him into the coal fields of Britain and eventually to fashionable offices in the West End of London, a Roman Catholic in matters of religion, and a writer whose first book was a success. He lived in England until World War II, left the country for the United States in the early 1940s, and had lived in Europe since 1955 and in a village near Montreux in recent years.

Like most writers, Mr. Cronin wrote about what he knew best. Unlike the books of most writers, his books sold in the millions. In 1961, it was estimated that sales of his novels in the Soviet Union alone had exceeded 3 million, though Mr. Cronin told Newsweek magazine that he had not received a single ruble in royalties. According to the same magazine, Mr. Cronin had found 22 publishers in the United States and elsewhere in the world.

The first of his books, "Hatter's Castle," appeared in 1931. Mr. Cronin wrote it in Scotland while recuperating from an ulcer. It is a story about a hatter in a small Scottish town who believes he is of noble birth and is destined for great things. Inevitably, his hopes collapse. Critical reception of the book ranged from praise for its "objective writing and rich and moving humanity" to damnation as the "uninspired epitome of Nineteenth-Century novels."

The public nonetheless bought it and read it. Mr. Cronin decided to devote his full time to writing.

In 1935, he published "The Stars Look Down," which was set in a mining community in the North of England from 1903 to 1933. Its central incident is a mine disaster and its central theme the awful conditions under which miners work. The New York Times said the author was "uncannily like Dickens" and hailed the work as one of the best novels of the year. It later was made into a motion picture.

Mr. Cronin's best-known books were "The Citadel," which appeared in 1937 and recounted the experiences of a young physician in a Welsh mining community, and "The Keys of the Kingdom," (1941) the story of a Catholic priest who is ordered by the church to do missionary work in Asia. There the central character, Father Chisholm, learns that "tolerance is the highest virture." The novel later was made into a film that starred Gregory Peck.

"The Citadel" made headlines for its bleak criticism of the British medical establishment of the day. One critic dismissed it as "dramatized pamphleteering." The British Medical Association was moved to reply to Mr. Cronin's charges. Mr. Cronin said, "The horrors and inequities detailed in the story I have personnally witnessed. This is not an attack against individuals, but against a system."

Mr. Cronin's other books included "The Green Years," "Shannon's Way" and "Pocketful of Rye." He also created "Dr. Finlay's Casebook," a story about two Scottish doctors sharing a practice, which became one of the longest-running series ever to appear on British television.

Archibald Joseph Cronin was born July 19, 1896, in Cardross, Scotland. He won a national essay contest as a boy, was educated at the University of Glasgow, and served in the British Navy during World War I.

For four years after the war, he practiced medicine in a Welsh mining village. In 1924, the Ministry of Mines gave him an appointment that enabled him to travel throughout the mining regions of Britain to study pulmonary ailments. He later moved to London and established a successful private practice.

"I had a miserable boyhood," Mr. Cronin said in an interview in 1964. "I was an unwanted child and we were very poor . . . . I don't care about money. It doesn't interest me at all. When I was young and wanting to get on I felt differently. But now when they ring me up and tell me that they have sold some right for so many thousands I don't pay much attention."

Mr. Cronin enjoyed collecting pictures. His other hobbies included salmon fishing, golf and gardening.

Mr. Cronin is survived by his wife, the former Agnes Mary Gibson, and their three sons.