A stricter drink-drive limit has come into force in Scotland, making it lower than the rest of the UK.
The limit north of the border is now 50mg of alcohol in 100ml of blood, down from 80mg.
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Holyrood's Justice Secretary Michael Matheson said the move would help save lives and reduce accidents on Scotland's roads.
Drivers south of the border have been warned they could legally get behind the wheel after having a drink in England, but find they are over the limit if they travel north into Scotland.
Four people were arrested across Scotland between the new law coming into force at one minute past midnight and 6am today.
In Glasgow, police carried out an operation stopping drivers in the Gorbals area as part of their annual festive drink-drive campaign.
Mr Matheson, who was at the operation, said: "The principle purpose behind this change is to make the roads in Scotland safer. One in 10 of all the fatalities that occur on our roads each year are associated with someone who is over the drink-drive limit.
"That's 20 lives that are lost, families that are devastated through the loss of their loved one as well, and by reducing the drink-drive limit we want to send a very strong message to drivers that we will not tolerate drink-driving and that you put yourself at risk of being caught drink-driving by this reduction in the drink-driving limit.
"I believe that will not only reduce accidents but also reduce fatalities on Scottish roads and in that way make Scottish roads safer."
He said that Scotland is leading the way across the UK and that the change will bring the country into line with the rest of Europe.
The lower limit has been brought in north of the border after Holyrood unanimously approved the move last month.
Police Scotland Assistant Chief Constable Bernard Higgins said: "The message for me is very clear. Every time you get behind the wheel having consumed alcohol you're presenting a risk to yourself, to other road users and to pedestrians, so don't do it, it's simply not worth it.
"Far too many people die or are injured each year on Scotland's roads as a result of drink-driving and the irresponsible disregard shown by drink-drivers for not only their own safety but that of other road users and pedestrians.
"Evidence from across Europe, where the lower limit already applies, suggests we will see reductions in drink-driving and a corresponding increase in lives saved."
He also warned people to think carefully about driving the morning after a night out if they have been drinking.
Almost eight in 10 (79%) Scottish motorists support the reduction, according to a new survey.
The RAC study also found more than a third (38%) of UK motorists living outside Scotland believe the alcohol limit for driving should be reduced to the same level throughout the rest of Britain.
The study of 2,607 motorists - including 10% from Scotland - found almost a quarter (23%) of UK drivers would prefer to go a step further and have a total ban on consuming any alcohol before driving.
While the limit in the rest of the UK will remain at 80mg of alcohol per 100ml of blood, campaigners called on Westminster to take a tougher approach against drink-driving.
Road safety organisation Brake is calling for a limit of just 20mg to be brought in.
AA president Edmund King said: "The different limits north and south of the border should not be a problem if drivers follow the mantra: If you are going to drive, don't drink, and if you are going to drink, don't drive. It is far too dangerous to try to judge how much you can safely drink whether the limit is 50mg or 80mg."
Campaigners at Scottish Health Action on Alcohol Problems have also welcomed the reduction.
Meanwhile, prosecutors have dropped a decades-old deal not to pursue drink-drivers who are just over the limit.
In a clear signal that that a tough new law introduced today will be vigorously enforced, the Crown Office has told lawyers it will no longer guarantee leeway.
Crown Agent Catherine Dyer has formally withdrawn from an undertaking made in 1983 to the Law Society of Scotland that drivers would only prosecuted if they were fully five microgrammes over the limit.
The old convention - designed to avoid courtroom quibbles about breathalysers and other equipment - has not been renewed.
Ms Dyer has told the Law Society drivers will now be prosecuted "where there is sufficient and reliable evidence and where it is in the public interest to do so".
However, lawyers handling drink-drive cases believe Ms Dyer's stance means that - in effect - the real drink-drive limit has fallen from 40 microgrammes, the old minimum for prosecution, to 22 microgrammes. And that is a drop of nearly 50 per cent.
The change was uncovered by "Unlockthelaw", a new legal advice website and detailed in a letter sent by Ms Dyer to the Law Society but not widely distributed outside legal circles.
Thomas Ross QC, one of Scotland's leading criminal advocates, criticised the Crown for ditching leeway without consultation.
He said: "Today the fallibility of the testing equipment and those operating it is recognised in two ways.
"The first is the citizen's right to replace the sample with blood or urine where there is a breath reading below 50 microgrammes.
"The second is the 1983 undertaking not to prosecute where there has been a breath reading of less than 40 microgrammes.
"The Government, after consultation, has preserved the first of those rights (where the breath reading is no more than 31 microgrammes). "The Crown, without any consultation at all, has completely withdrawn the second."
Prosecutors, insiders stress, will still use their discretion in deciding whether to go after borderline cases.
Veteran solicitor Peter Lockhart of Ayr, a member of the Law Society's criminal law committee, does not believe the lack of leeway will help with appeals or defences. Mr Lockhart said: "The law is the law."
The new drink-drive limit has been widely welcomed with a poll last week suggesting two-thirds of voters in England and Wales would like to see Scottish-style reforms.
Justice Secretary Michael Matheson said: "This change will bring Scotland into line with most of Europe. It's not about criminalising drivers, it's about making our roads safer and sending a clear message that even one alcoholic drink will affect the ability to drive."