A mental health trust had no adult beds available on two occasions in the past week, with more than 30 people waiting for a place, the BBC understands.
Some patients were kept in emergency departments in Norfolk and Suffolk for up to a week while they waited to be admitted to a specialist unit.
Norfolk and Suffolk NHS Foundation Trust (NSFT) said it had “been facing increasing demand for inpatient care”.
But campaigners said there had not been enough beds for a number of years.
An inspection report earlier this year gave the trust its fourth inadequate rating in six years.
Mark Harrison, from the Campaign to Save Mental Health Services in Norfolk and Suffolk, said the issue of a lack of beds was “vitally important” and had a knock-on effect for hospitals.
He said those who needed mental health beds but who could not get one might be left with their families or go to accident and emergency departments at hospital.
“A&E is full of people in severe mental health distress because there aren’t mental health beds,” he said.
NSFT is creating an extra 15 beds in the next year at Hellesdon Hospital, but Mr Harrison said that was not enough to help.
Alex Stewart, chief executive of Healthwatch Norfolk, said: “We are concerned about the recent pressure on bed space.
“We will talk to NSFT to seek reassurance that this is only a very temporary issue.”
Stuart Richardson, chief executive officer at NSFT, said increasing demand had “led to difficulties with capacity due to the acuity of the patients who need our support and well-documented pressures across the wider system”.
He said: “We have been working closely with our health and care partners to free up capacity wherever possible and have seen some improvements as a result.
“We are keeping the situation under daily review and will continue to do everything we can to ensure our patients can access safe and timely care.”
The trust added it was still placing patients out of the area “where necessary”.
Analysis by Nikki Fox, BBC Look East health correspondent
The fact that more than 30 people in Norfolk and Suffolk have been without the mental health bed they need proves that the problems in the emergency care system are not just about physical health.
At one point seven patients were waiting for a mental health bed at the Norfolk and Norwich Hospital.
This not only puts pressure on hospital staff but takes up beds that are needed for people waiting for physical healthcare.
In the past, it would be unusual for so many people to be waiting for mental health beds because patients would be sent out of area to be kept safe.
With trusts told they should avoid that and everywhere struggling with increases in demand, it means families and hospitals are being put under more pressure to look after those with severe mental health needs.
Source: BBC