The NHS Greater Glasgow and Clyde board said only people with a “very urgent or life-threatening condition” should attend as all its A&E facilities were “operating well over normal capacity”.
Mr Yousaf warned “this is going to be an extremely challenging period” and NHS boards across Scotland were “already making exceptionally difficult decisions in terms of prioritising care”.
Earlier this week, Scotland recorded a record high number of cases for the pandemic, with 20,217 infections reported on Monday.
Although the daily total has since fallen back, Mr Yousaf said Scotland is “absolutely heading into that peak”, adding this is expected to happen “over the course of the next few weeks”.
Dr Scott Davidson, the deputy medical director for acute services at NHS Greater Glasgow and Clyde, said: “As a result of current demand at A&E, people are facing long wait times and our staff are under severe pressure.”
Urging people to attend minor injuries units or GP practices instead, he said: “Unless very urgent or life-threatening, please do not attend our A&Es unless you are referred.”
Doctors are ‘beyond breaking point’
Dr Sandesh Gulhane, a GP and the Scottish Tories’ shadow health secretary, said: “This is damning evidence that the SNP Government failed to prepare our health service for the toughest winter they have ever faced.
“My colleagues on the frontline are beyond breaking point and have suffered due to Humza Yousaf’s inaction and lack of planning ahead of the winter period.”
Jackie Baillie, Scottish Labour’s health spokesman, said the rise of cases in care homes was “an incredibly worrying development” after the virus killed scores of residents in the first wave.
The Scottish Retail Consortium disclosed that footfall on the country’s high streets last month fell by 22.8 per cent compared to December 2019 and by three per cent compared to the previous month.
This was the fifth consecutive month that Scotland experienced the sharpest drop in shoppers out of any of the home nations, despite December being traditionally the busiest month of the year.