The five-day relaxation of rules over Christmas has been cancelled - with households only able to meet up on December 25.
It comes as concerns rise over a new variant of coronavirus - which is said to be up to 70% more transmissible than the original strain.
It means people can still meet in up to three households, but only for that 24-hour period - and the first minister is urging people to only do so if it is essential.
Meanwhile level four restrictions will apply to all of mainland Scotland for a period of three weeks from Boxing Day morning.
And the school holiday will be extended until January 11 - and at least a further week of online learning after that period.
Nicola Sturgeon has also announced a "strict travel ban" between Scotland and the rest of the UK throughout the festive period. She says she will be asking police and transport authorities how enforcement of this can be strengthened in the coming days.
She said: “Last week it was confirmed that a new variant of the virus had been identified in the UK, and I advised Parliament that a small number of cases of it had already been identified in Scotland.
“While further analysis is required to establish this one way or another, we already have a concern that this strain may be driving what appears to be faster transmission in some hospitals and care homes.
“That’s why we have decided to act and to act firmly.
“Firstly, we are asking everyone to redouble your personal efforts in sticking to the rules and following FACTS.
“Secondly, we intend to maintain a strict travel ban between Scotland and the rest of the UK. This will remain in place throughout the festive period. We simply cannot risk more of this strain entering the country if we can possibly avoid it.
“Thirdly, we now intend to change the law to allow mixing indoors in a bubble on Christmas Day only. The household limits will still apply - a maximum of 8 people from 3 households - is the law. But the advice will be to minimise numbers as far as possible.
“My message is stay home, stay safe and enjoy Christmas. That is in your own interest and the interest of everyone you love.”
Speaking about beyond Christmas, the first minister said: “To limit, as far as we can, the risk of this strain spreading further than it has within Scotland, we intend to apply Level 4 measures to all of mainland Scotland for a period of three weeks from one minute after midnight on Boxing Day morning.
“The only exceptions will be Orkney, Shetland and the Western Isles, and the other island communities where we have reduced restrictions in recent weeks, who will go into Level 3 but with strict restrictions on who can travel to and from those islands.
“The situation the new strain presents means that until we are sure it is firmly under control – until we are sure we are not facing the same situation as the south of England today faces, we must slightly change our plans for the way our schools work.
“Schools still open next week will close as planned – we are not changing that. They were then due to reopen from 5 January with all councils areas back by the 7.
“Instead of that, here is what we are ow going to do - for the children of key workers – such as nurses in our hospitals – schools will open and stay open as normal. They will also open as normal for the most vulnerable children. All teachers will also return to work as scheduled and planned. For the majority of pupils, however, the holidays are being extended until 11 January. Starting on that date, learning will be online until at least 18 January.
“After that, assuming we are confident we have the virus under control we will aim to reopen schools more fully but at least until then, schools will go online only other than for the children of key workers and the most vulnerable.”
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