Aside from a small number of exemptions, it is illegal to enter or leave a Tier 4 area, which covers 16 million people across London, the South East and East of England.
The government say these strict restrictions are needed to stop the spread of the faster-spreading mutant coronavirus strain.
In a statement on Sunday morning, Transport Secretary Grant Shapps said: “It is incredibly important that people follow the guidance, stay at home and do not attempt to travel.
“Our focus must be stopping the spread of this virus, protecting lives and our NHS.
“If you are in Tier 4, the law means you must stay at home and you cannot stay overnight away from home. Across the rest of the country, you must stay local.”
Mr Shapps added: “Follow the guidance and please do not come to a station unless you are permitted to travel.
“Extra BTP officers are being deployed to ensure only those who need to take essential journeys can travel safely.”
Ahead of the introduction of the new restrictions at midnight, Saturday evening saw chaotic scenes at major London train stations as people left the capital.
People were told that social distancing “will not be possible” due to the volume of travellers on board, and those that felt “uncomfortable” were advised not to stay on the train.