Harvard Police Lt. Lawrence J. Murphy has replaced Chief Paul E. Johnson as the officer-in-charge of the University's police force, officers said yesterday.

Johnson, who police sources say is seriously ill, spent a week at New England Deaconess Hospital in Boston earlier this month, according to the hospital. On Tuesday, General Counsel Margaret H. Marshall informed police personnel of the decision to temporarily install Murphy as officer-in-charge, according to Murphy.

Murphy said the chief will resume his duties after he completes medical treatment. The lieutenant said there was no timetable for Johnson's return.

"The general counsel has decided that there should be someone to temporarily step in and take his duties," Murphy said.

Sources said Johnson had spoken with Marshall Sunday about his illness. In an interview on Tuesday, Marshall said she often met with the chief, but she would not confirm or deny whether she had spoken with Johnson on Sunday.

The change in management comes at a precipitous time for the chief and the Harvard police. Johnson, a former Boston police area commander in Roxbury and Mattapan, has come under fire from security guards and some police officers for a hands-off management style some say has led to turmoil in the guard unit and throughout the department.

Four police department employees, interviewed on condition of anonymity, speculated yesterday that Johnson might not return to his job. In the past, the chief had told associates he would retire at the end of this year, when he completes his tenth year on the job.

Officers said the decision to give the temporary assignment to Murphy was not a surprise. Murphy is currently in charge of special operations, including VIP protection, for the department.

The new job will add significantly to Murphy's duties, Murphy said