Non-urgent surgery and outpatient appointments are being delayed but serious operations will go ahead.
The major incident was declared in Essex in response to a "significant growing demand" on hospitals. It will enable local leaders to seek further government support, the Essex Resilience Forum (ERF) said.
Hospital bosses in Southend and Basildon also made a plea for any clinical staff who are on annual leave to return to work immediately to relieve the pressure.
The ERF - which is made up of members of the NHS, emergency services and local authorities - added that the number of patients in Essex receiving treatment for