Scots are to be ordered to stay at home amid a fresh Covid-19 lockdown which will see schools remain closed to pupils until February.
First Minister Nicola Sturgeon said new curbs would be introduced at midnight in a bid to contain the new, faster-spreading strain of the virus.
New laws will require people to stay at home and work from home where possible.
Outdoor gatherings are also to be cut back, with people only allowed to meet one person from one other household.
Places of worship are to be closed, group exercise banned, and schools will largely operate via online and remote learning.
These rules will apply across the Scottish mainland until at least the end of January, and will be kept under review.
Island areas will remain in level three - but Ms Sturgeon said they would be monitored carefully.
Prime Minister Boris Johnson later announced similar lockdown measures for the whole of England with all schools and colleges closing to most pupils until mid February.
A further 1,905 new cases were reported in Scotland on Monday - with 15% of tests returning a positive result, something Ms Sturgeon said "illustrates the severity and urgency of the situation".
The first minister said she was "more concerned about the situation we face now than I have been at any time since March last year", with the new coronavirus strain now accounting for half of new cases.
And she said a "steeply rising trend of infections" was threatening to put "significant pressure" on NHS services, saying hospitals could breach capacity within three to four weeks.
The new rules - which will be put down in law - mean Scots will only be allowed to leave home for essential purposes, such as shopping for food and medicine, exercise and caring responsibilities.
No limit is to be put on how many times people can go out to exercise, but outdoor meetings are to be limited to a maximum of two people from two households.
Everyone who can work from home will be required to, and people in the "shielding" category are advised not to go in to work at all.
The construction and manufacturing industries will remain open, but Ms Sturgeon said this would be kept under review.
Places of worship are to close, the number of people who can attend weddings is to be cut to five, and funeral wakes will no longer be allowed.
Schools are to remain closed to the majority of pupils until February, with Ms Sturgeon saying community transmission of the virus must be brought to a lower level amid concerns that the new variant of the virus spreads more easily among young people.
She said she knew remote learning presented "significant challenges" for parents, teachers and pupils, adding: "I want to be clear that it remains our priority to get school buildings open again for all pupils are quickly as possible and then keep them open."
The first minister said she was considering whether teachers could be given the Covid-19 vaccine as a priority.
More than 100,000 people have been given a first dose of the vaccine in Scotland, and the government expects to have access to just over 900,000 doses by the end of January.
However Ms Sturgeon said the best way to get schools open again was to drive down transmission of the virus - urging Scots to abide by the rules.
These are the toughest restrictions Scotland has faced since the lockdown of March 2020.
It is - once again - becoming compulsory to stay at home except for essential purposes like food shopping, exercise and medical care.
The extended closure of schools to most pupils is something the Scottish government was particularly keen to avoid.
These decisions are a measure of how worried ministers are about the rapid spread of the new variant of coronavirus, which is fast becoming the dominant strain.
With 225 cases per 100,000 people, Scotland is thought to be about four weeks behind London, which already has four times as many cases and NHS services under considerable pressure.
The Scottish government believes that without further action the NHS here would run out of beds for Covid patients within a month.
This new alert comes at the start of a new year which also brings new hope for a route out of the pandemic with two vaccines now beginning to offer protection.
Around 100,000 doses have already been administered in Scotland but it is likely to take several months to reach all in the most vulnerable groups.
The first minister said Scotland was now in "a race between the vaccine and the virus".
She said: "The Scottish government will do everything we can to speed up distribution of the vaccine. But all of us must do everything we can to slow down the spread of the virus.
"We can already see - by looking at infection rates in the south of England - some of what could happen here in Scotland. To prevent that, we need to act immediately and firmly.
"For government, that means introducing tough measures - as we have done today. And for all of us, it means sticking to the rules."
'High quality education'
Scottish Conservative group leader Ruth Davidson raised concerns about online learning, saying it was vital that pupils had "equal access to high-quality education".
And Scottish Labour leader Richard Leonard said teachers and working parents would need support to make the remote learning system work.
Ms Sturgeon said her government had "agonised" over the decision on schools, and said the "fundamental priority" was to re-open them in full as soon as possible.
She said: "Just as the last places we ever want to close are schools and nurseries - so it is the case that schools and nurseries will be the first places we want to reopen as we re-emerge from this latest lockdown."
The NHS has coped so far in Scotland - more so than many other parts of the UK.
But in places like Glasgow and Lanarkshire it has been very, very tight. And here like everywhere else staff are bracing themselves for the post-Christmas effects of rising cases.
The first minister gave some stark figures on hospital and ICU occupancy - suggesting we are just weeks away from reaching limits.
There is so little give in the system they will be glad to see everything possible done to prevent stretched services being overwhelmed at a time when we are on our way to getting out the other side.
There is real anxiety about what the next few weeks might bring.