NHS bosses made the decision following the sustained pressure experienced by Cornwall's main hospital and emergency department over the week.
Kate Shields, the chief executive of NHS Cornwall and Isles of Scilly Integrated Care Board, said: "We are currently seeing a high number of ambulances waiting outside the Royal Cornwall Hospital and of patients in our emergency department.
“There are also a large number of patients in Cornwall’s hospitals who are medically fit to be discharged but are awaiting the right care.
"Declaring a critical incident allows the NHS to take additional, immediate steps to create capacity, help discharge patients, relieve pressure on our emergency department and release ambulances and their crews.
“Our staff right across the county are working exceptionally hard to provide care based on peoples' medical need.
"We urgently need the public’s support to make sure we can see those who are sickest and have the greatest need.
"Please only come to the emergency department if you have a life or limb-threatening emergency. Other NHS services are available, and we encourage you to use them whenever possible. These include our network of minor injury units, our urgent treatment centre at West Cornwall Hospital, out-of-hours GP services, local pharmacies and NHS 111.
“People who come to the emergency department but can be treated elsewhere will be directed to those services.
“Families, friends and neighbours are urged to help us too by offering to support someone waiting for home care to leave hospital. We ask them to contact the ward directly if they can help in any way.
"Fast access one-off grants are also available, to use towards paying for care, expenses or equipment to help you get a relative or friend home from hospital. If you could support someone in your home, or in their own home, with a little extra help, please speak to the nurse in charge on the ward.
“Getting someone home a day or two sooner will mean we can free up a vital hospital bed for someone else in urgent need – please help us if you can."
If you are unwell with flu, norovirus, COVID-19 Please don’t go to a health setting such as the emergency department and our minor injury units or visit a loved one in hospital. Please use the NHS 111 online service first for advice. You can help us by not spreading your illness to others, instead stay home, get plenty of rest and stay hydrated.
Have a loved one in hospital? Please be ready to support the home from hospital and don’t visit if you are unwell. Staff on the ward can advise on how you can help – including providing one off financial payments if needed.
Look up ‘where is best’ advice online