A troubled health board will come under more direct Welsh government control after a report found "institutional abuse" at a mental health unit.
The Tawel Fan ward at Glan Clwyd Hospital, Denbighshire, closed in 2013.
Health Minister Mark Drakeford said the move to special measures reflected "serious and outstanding concerns" about the leadership at Betsi Cadwaladr University Health Board in north Wales.
Its chairman said the board would co-operate fully to improve matters.
In the report, by health specialist Donna Ockenden, relatives said patients on the Tawel Fan ward were treated like animals in a zoo.
The decision followed a meeting involving the Welsh government, Healthcare Inspectorate Wales and the auditor general on Monday.
It is the first time a health board in Wales has been placed in special measures, the highest of three levels of intervention.
First Minister Carwyn Jones said the government and people in north Wales "have had enough".
"They (the health board) have had enough opportunity to regain the trust of the people of north Wales so we had to take these measures in order that the board can be rebuilt," he said.
"Tawel Fan happened two years ago but we weren't aware of the full picture... the board has lost the support of local people and we can't let that happen anymore and now is the time to rebuild."
In a statement, Mr Drakeford said: "A thorough and balanced assessment has taken place on areas of concern that will form the basis of actions to be taken as a result of special measures."
He added: "Whilst special measures will apply to the organisation, I wish to reassure both patients and communities served by the health board and staff working for it that day to day services and activities will continue as normal."
Further details of what action would be taken in practice will be given by the minister to assembly members on Tuesday.
In response, board chairman Dr Peter Higson said: "I recognise the gravity of the situation and the need for swift remedial action.
"I will ensure that the health board and its officers will work and co-operate fully with the Welsh government in achieving the necessary improvements for the benefit of the patients and public in north Wales."
Analysis: Nick Servini, BBC Wales political editor
So, with 11 months to go until the assembly elections, the Welsh government has taken direct control of the biggest health body in Wales.
In one fell swoop, the state of the NHS just became an even bigger political issue than it was already.
Is it a huge gamble or an opportunity for ministers to try to change the narrative of problems in health services across north Wales?
Behind the scenes, it has been stressed the special measures are not a direct result of the scandal at the Tawel Fan mental health ward.
Instead, that was one of a number of problems that contributed to a loss of public confidence in the board.
Whatever the reason, this changes the nature of the debate.
Up until this point, the blame for failures could at least have been shared between ministers and local health managers.
That division no longer exists.
On the flip side, this is simply the right thing to do, regardless of political calculations, and will be portrayed as such by the Welsh government keen to show it is taking charge of matters.
The Conservatives' Shadow Health Minister Darren Millar said: "While I welcome the decision to place this dysfunctional crisis-hit health board in special measures, the move is long overdue and something I've been calling for since governance problems were first identified back in 2013.
"Placing the health board in special measures is the first step to turning the organisation into an effective provider of first class health care, where patient safety comes first."
Mr Millar also called for an independent inquiry into the Welsh NHS, blaming budget cuts by Labour ministers for contributing to management failings.
Aled Roberts of the Welsh Liberal Democrats said: "I am hugely disappointed that the situation here in North Wales has deteriorated to the extent that this drastic decision has needed to be taken. I do, however, believe it is the right decision.
"This is a health board in desperate need of help."
Plaid Cymru AM Llyr Gruffydd said there had been "a failure of political leadership regarding Betsi Cadwaladr for some years now", claiming the Welsh Labour government had "washed its hands of the problem for too long".
"Apologies mean nothing without a change of direction and we're not seeing that with this health board," he added.
Earlier on Monday, Ms Ockenden told BBC Radio Wales her report had uncovered "a terrible scandal" and "a stain on the NHS in Wales".
She said the families had "run out of patience" and expected "fair, honest and transparent action in a timely way".
Several members of staff on the ward are facing disciplinary procedures but North Wales Police decided not to pursue criminal charges after investigating allegations of mistreatment.
Since the report was published, opposition parties and local health watchdogs have called for ministers to take action and for senior heads to roll.
In June 2013, the then chairman and chief executive of the Betsi Cadwaladr board resigned after a report condemned management failings and financial problems across its operations.