Police have said crime has fallen by 28% since Britain was locked down to battle the coronavirus.

Young people have been the biggest flouters of tough restrictions on gatherings, as the hot weather has led to defiance of emergency laws.

In their most detailed picture yet of crime since laws came in to keep Britons apart, police also revealed that drug supply might be falling, with criminals posing as key workers to deal and targeting long supermarket queues to push drugs.

The government rushed in emergency powers three weeks ago to enforce the lockdown, restricting when people could leave their homes and limiting gatherings to two people.

Some mistakes have emerged as officers struggled to apply the new laws. Police on Wednesday admitted that 39 children had been wrongly fined for violations of restrictions. Police said those fines had been rescinded because the emergency laws do not allow them to fine those aged under 18.

Falls in crime recorded by police in the four weeks up until 12 April included a 37% drop in burglary, a 27% drop in vehicle crime, serious assault and personal robbery. Reported rape offences fell by 37% and shoplifting fell by 54%, with non-essential stores closed.

The National Police Chiefs Council (NPCC), which issued the figures, said forces in England and Wales had seen a 3% increase in reports of domestic violence and were braced for further rises. The rise picked up so far by police is smaller than that detected by charities.

Police said there had been a 59% rise in reports of antisocial behaviour, which they believed was linked to reports about alleged breaches of the emergency laws.

Enforcing the lockdown rules had led to more than 3,203 fines being issued, with one given to a person aged 100, but it was clear that most fines were being issued to the young.

One-third of fines were issued to those aged 18-24, with another third to those aged 25-34.

Deputy chief constable Sara Glen, from the NPCC team working on the coronavirus, said younger Britons were the group struggling most to stick to the lockdown rules: “If we have good weather people naturally want to be outside. So, this is something which is a real challenge to us with our demographics in counties where they’ve got a younger population who want to party, who want to have barbecues, who want to be outside when it’s good weather.”

Police stressed that the vast majority were obeying laws in the first place and that they had had more than 100,000 “chats” with people they suspected were breaking them, with the vast majority complying or not needing to have the emergency laws used against them.

Some police forces appeared from data issued by the NPCC to be issuing fines more frequently than others. Lancashire issued 380 fines, possibly linked to having popular sites such as Blackpool, while Humberside only issued two fines. Hot weather played a part, police believe, in 424 fines being issued last Saturday to those flouting the rules to enjoy the sunshine over the long Easter bank holiday. But by Monday, as the weather cooled, only 177 fines were issued.

Serious and organised crime was also adapting, said Lynne Owens, the director general of the National Crime Agency.

She said a van driver had been caught on Tuesday near Calais with face masks concealing 14kg of cocaine. With British streets empty, drug dealers were posing as key workers to offer their commodities and even targeting long queues snaking around supermarket car parks to try to deal drugs, the woman in charge of fighting serious and organised crime said.

“They are having to find new ways of working and new networks,” Owens said.

“Drug dealers moving illicit drugs are concerned about greater scrutiny as they recognise that, with less people on the streets, they are more visible.

Of course, they will be looking at different opportunities, wearing high-vis clothing so they start to look like key workers, deploying or dealing from supermarket car parks where there may be more people around.”

The NPCC chair, Martin Hewitt, said some mistakes had been made, but they were few and understandable given that officers were under pressure, trying to enact laws restricting people’s usually lawful behaviour in a way they had never had to before: “Of course there have been mistakes and I think we have been very quick to come forward when we have made mistakes.

“But I would like to think that the public would have some recognition of the fact that this is legislation that came in at high speed a few weeks ago, is highly, highly unusual and we are having to adapt to that across the whole of the service.”