Many streets and stations were deserted during what is normally rush hour - though some trains were still full and cars were on the roads.
Under the new measures announced by the Government, people will only be allowed out of their homes for:
shopping for basic necessities, as infrequently as possible;
one form of exercise a day - for example a run, walk, or cycle - alone or with members of your household;
any medical need, to provide care or to help a vulnerable person;
and travelling to and from work, but only where this is absolutely necessary and cannot be done from home.
The city centre was virtually deserted as non-essential shops were also ordered to close by Mr Johnson.
In Bristol meanwhile, Temple Meads station was almost entirely empty.
In Essex, dozens appear to be queuing outside Costco ahead of what is set to be a long period of self-isolation.
That's despite Government advice that there is no need to stockpile food.
In London, usually bustling with commuters and traffic jams during rush hour, the streets and stations were strangely quiet.
Waterloo Station, usually one of the busiest in the country, was eerily quiet.
But not everyone had adhered to Mr Johnson’s declaration and many still have to travel for essential work.
On London's tube network, pictures have been shared of crammed carriages as a reduced service has left fewer trains for commuters to spread out on and sit or stand at least 2m apart from each other.
Nurse Julia Harris, who commutes to work at Imperial College NHS Trust, said she had left earlier and changed her route in a bid to avoid crowds but still found services busy.
She said: “Seats on the train all had at least one person so people needed to stand, and the District line was busy as well.
"I still don’t think things have improved as a large amount of people are commuting early in the morning.
“It is concerning because I have to come to work.
"The choice isn’t there and my commute is quite long.
"I worry for my health more on my commute than actually being in the hospital.”
Ms Harris said the reduction in Transport for London services meant “you now have more people waiting and piling onto the Tubes and trains”.
“The issue is key workers aren’t just health professionals – I think we under-estimated how many people are needed to keep things running.”
In response to pictures of crowded transport, London Mayor Sadiq Khan said “we must stop all non-essential travel”.
Mr Khan demanded that employers enable their staff to work from home “unless it’s absolutely necessary”, adding: “Ignoring these rules means more lives lost.”
He added a growing a number of staff are off sick or self-isolating, meaning Transport for London is unable to run more services.
Vernon Everitt, TfL’s managing director of customers, communication and technology, said: “To save lives, everyone must follow the Government and Mayor’s instructions to stay at home and only travel if absolutely essential.
"Only critical workers should be using public transport and no one else.
“We thank those acting on that which has brought passenger journeys on the Tube down by around 87 per cent.
"But to save lives and enable critical NHS and other workers to use our reduced services, more people need to stop travelling right now.”
Elsewhere in the UK
Coronavirus: Everything you need to know