KUALA LUMPUR, Malaysia—A senior opposition lawmaker was detained Tuesday for divulging details from a classified auditor general’s report on troubled state investment fund 1Malaysia Development Bhd., or 1MDB.

Malaysia’s police chief confirmed that the arrest of Rafizi Ramli, vice president of the Parti Keadilan Rakyat party, was carried out under the Official Secrets Act and that he would be held overnight. It was unclear when he would be released. (Update: Malaysian Opposition Figure to be Held for Three Days in 1MDB Secrets...