What are the new Christmas Covid rules?
Mixing allowed on Christmas Day only
Plans to allow Scots to celebrate Christmas with other people by forming "bubbles" are being scaled back and the tougher restrictions will come into force across the country from Boxing Day.
The news follows confirmation from scientists of a faster-spreading coronavirus strain with 17 cases of Covid-19 mutation identified in Scotland so far.
Here is an outline of what the new Christmas rules are, and what new restrictions are coming into force.
What do the changes mean for Christmas Day?
The plans for a UK-wide relaxation of the restrictions from December 23-27 have now been scrapped.
Instead, up to eight people from a maximum of three households will still be able to gather indoors on Christmas Day itself.
However, no travel in or out of Scotland is allowed because a "strict travel ban'' will be in place.
Nicola Sturgeon has also urged Scots to only meet with other households "if you really, truly need to''.
There will be no changes to the current restrictions until Christmas Day.
What happens after Christmas Day?
On Boxing Day, the whole of mainland Scotland will be placed into the Level 4 restrictions - the highest of the five tiers - for at least three weeks.
Non-essential shops, pubs and restaurants will have to shut except for takeaways, drive-throughs and deliveries.
Only essential travel will be permitted.
The Scottish islands - Orkney, Shetland and the Western Isles - will be placed in Level 3, with greater restrictions on gatherings and hospitality.
The lockdown will be reviewed after two weeks, Ms Sturgeon has said.
School holidays extended
Schools will continue as normal for the final few days of this term until the start of the Christmas break.
The Christmas holiday is now being extended until January 11 for the majority of pupils, although schools will still reopen as scheduled for the most vulnerable and children of key workers.
But teaching will be done online for at least the first week of the new term, Ms Sturgeon has announced.
She said: "After that - assuming we are confident we have the virus under control - we will aim to reopen schools fully.
"But at least until the 18 January, schools will go online only other than for the children of key workers and the most vulnerable.''
New type of Covid-19 found
The move comes after scientists on the Government's New and Emerging Respiratory Virus Threats Advisory Group (NervTag) concluded that the mutant strain identified by Public Health England - known as VUI2020/01 - was spreading more quickly.
Ms Sturgeon said it may be up to 70% more transmissible than the old variant.
Speaking at an emergency coronavirus briefing on Saturday evening, Ms Sturgeon said the new strain of the virus is the "most serious and potentially dangerous juncture'' faced since the start of the pandemic.
She added: "If it wasn't for this new strain, I wouldn't be standing here right now announcing this because our case levels right now - because of the Levels approach we've taken - are actually quite stable.
"This is preventative action because we know there is a more-rapidly transmitting strain out there and so we're acting preventatively.
"Everything we've learned about this virus says that you do that - you act quickly and don't have regrets later, and that's what we're seeking to do.''
What will happen if people break the rules?
Police Scotland's Assistant Chief Constable Alan Speirs said enforcement of the travel restrictions would continue to be a "last resort where there is a clear breach of the legislation''.
He said the force would not be setting up road blocks or carrying out spot checks but added: "Where travel restrictions apply, officers will continue to use the common sense, discretion and excellent judgment that they have applied since the crisis began.''
Current restrictions mean it is illegal to travel into or out of council areas in Level 3 or Level 4 without a valid exemption.
Police have the power to issue £60 fines to rule-breakers, although these are halved to £30 if paid within 28 days.
Repeat offenders can face penalties of up to £960.
Are other parts of the UK changing their rules?
Yes, England and Wales have both strengthened their restrictions.
Prime Minister Boris Johnson announced London, the South East and East of England will move to a new Tier 4, effectively returning to the lockdown rules of November, and will not be able to join or form Christmas bubbles.
For everyone else in England, the Christmas "bubble'' policy - allowing up to three households to meet up over the holiday period - will be severely curtailed to apply on Christmas Day only.
The Welsh Government has announced that lockdown restrictions will come into effect in Wales from midnight on Saturday, while festive bubbles will only apply on Christmas Day.
First Minister Mark Drakeford said the rules would mean non-essential retail, close contact services, gyms and leisure centres and hospitality would close at end of trading'' on Saturday.
Stay-at-home rules will also come into effect from midnight.
So far there has been no announcement of changes to the Christmas Covid-19 restrictions in Northern Ireland, despite rules being tightened in other parts of the UK.
Sources said meetings between the Executive and the UK Government had been ongoing all day.