Places at Scotland's high-security Carstairs state hospital will be cut from 240 to 140, the government said.
Health Secretary Nicola Sturgeon hit out at "scaremongering" over the plan, insisting patients would be transferred only after a proper risk assessment.
The creation of two medium secure units in Glasgow and Edinburgh will allow the bed reduction at Carstairs, she told the Scottish Parliament.
Opposition parties sought a number of assurances over the changes.
Ms Sturgeon told MSPs: "It is very important to stress this redevelopment will not, as some have rather irresponsibly suggested, result in some sudden exodus of 100 patients from the state hospital."
Patient transfer
Ministers will retain the power to decide on the transfer of "restricted" patients - of which there are currently 115 in Carstairs.
Patients can launch a court appeal if they feel they are being held in conditions of excessive security.
"With modern treatment, both through drugs and therapies, their mental illness can be treated and managed and the level of risk they present reduced," Ms Sturgeon said.
"In such circumstances it is appropriate that they be considered for a move to lower security."
The Orchard Clinic, which opened in 2000 in Edinburgh, provides 50 medium secure beds, while Glasgow's 74-place Rowanbank Clinic opened last year.
Labour health spokesman Margaret Curran sought assurances that no transfer would be delayed by a lack of resources, while the Tories' Mary Scanlon said that no patients should be moved into communities to make room in medium security facilities for Carstairs patients.
Ross Finnie, the Liberal Democrat health spokesman, questioned if enough attention had been paid to the transfer of patients from medium to lower level security facilities.
Recent Carstairs patients include John Johnstone, who raped and killed a young woman a decade ago in the Royston area of Glasgow.
Michael Clark, who ran over his brother in a tractor in Ellon, Aberdeenshire, and Joseph Boyle, who knifed a man to death in Glasgow, were also sent there.
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