Mr. Wole Soyinka, the Nigerian playwright and head of the drama department at Ibadan University, has been detained by the federal Government for security reasons. He was arrested after being interrogated by the federal police. Doctors, including Soyinka's personal physician, are quoted as reporting that he was not manhandled by the Nigerian police, as alleged by some foreign newspapers. His wife was allowed to visit him last Friday. News of other writers in the former Eastern Nigeria is obscure, although it is known that the poet Christopher Okigbo, the representative of Cambridge University Press here, was killed in action as an army major. Mr. Soyinka was reported in The Times last week to have been released by the federal authorities after his detention on August 17. Reports reaching London said that he was seriously ill. His arrest on that occasion followed the publication in Nigeria of a letter in which he urged the federal Government to call a truce on hostilities with the secessionist Eastern Region. Mr. Soyinka, who was educated at Leeds University, is the best-known poet, playwright, and novelist in Nigeria. Among his plays performed in Britain are, The Road, The Trials of Brother Jero, and The Lion and the Jewel.