Londoners who crowded on to trains to leave the capital on Saturday night were "totally irresponsible", the health secretary has said.
London has moved into tier-four restrictions, meaning it is illegal to leave the capital from 00.01 GMT Sunday unless for essential travel.
Footage shared on social media showed large crowds at St Pancras station.
Matt Hancock called for people to act responsibly but said the majority of people were abiding by the guidelines.
The announcement of the new rules, shortly after 16.00 GMT on Saturday, prompted a rush to London's railway stations.
By 19.00, there were no tickets available online from several stations including Paddington, Kings Cross and Euston.
Last train out of Saigon. Queue at St Pancras as we wait to board the Leeds bound train. pic.twitter.com/cFDBDNnYFC— Harriet Clugston (@HarrietClugston) December 19, 2020
At St Pancras, which connects south-east England with the East Midlands and North, several passengers told the BBC they were catching pre-booked trains and had not heard about the new rules being brought in.
However one passenger, who did not wish to be named, said she and her partner had made the "split decision" to take their young son to her parents' home on the coast.
"We just made the decision to leave based on the fact that my parents said come, and we couldn't bear the thought of no fresh air and a toddler going rogue round a small flat for the foreseeable," she said.
East Midlands Railway said it had run a full service on Saturday and only two of its 10 evening departures had been full.
A spokesman for the railway apologised for any "inconvenience and discomfort caused" by the "unexpected surge in passengers".
Some 21 million people are now under tier-four restrictions which ban households from mixing as well as requiring non-essential shops and businesses to close.
Mayor of London Sadiq Khan criticised the government for making the announcement "late", following days of reassurances restrictions would be relaxed over the Christmas period.
He said crowded stations were "a direct consequence of the chaotic way the announcement was made".
Mr Khan urged people in London and the South East to stay at home and avoid "breaking the rules".
He said: "Yesterday, technically speaking, you may not have been breaking the rules but you may well have the virus and not realise you've got the virus.
"Keep [the virus] within London, follow the rules and let's get on top of this."
Mr Hancock said only "relatively small numbers" were breaking the rules.
"The large, vast majority of people throughout this whole pandemic have followed the rules," he said.
He added people should "restrict social contact as much as is possible because this is deadly serious".