Nearly 600 people suspected of involvement in drug dealing have been arrested in a single week, police have said, as officers launch a national crackdown on so-called county lines gangs.

The operation focused on the networks that recruit children and vulnerable adults to export the illegal drugs trade from the UK’s cities to its more rural areas.

Officers took action to protect more than 900 people, including more than 350 children, and helped more than 30 people thought to be at risk of human trafficking and modern slavery. The drive also resulted in the seizure of more than £300,000 in cash and 46 weapons, the National Crime Agency (NCA) said.

“Tackling county lines and the misery it causes is a national law enforcement priority and these results demonstrate the power of a whole-system response to a complex problem that we’re seeing in every area of the UK,” said the NCA’s director of investigations, Nikki Holland.

“We know that criminal networks use high levels of violence, exploitation and abuse to ensure compliance from the vulnerable people they employ to do the day-to-day drug supply activity.”

Research released by the NCA earlier this year hinted at the scale of the trade, with nearly three times the number of phone numbers linked to it being identified than were previously known about. The agency’s assessment valued the illicit trade at about £500m per year and linked it to murder and sexual exploitation.

On Tuesday, the NCA said it had seized £176,780 worth of cocaine, £36,550 worth of crack cocaine and £17,950 worth of heroin in the operation, which ran for a week to Monday.

In one case, police involved in the operation found a 14-year-old missing girl from Cambridge in London and arrested three men on suspicion of offences linked to child sexual abuse.

In another, officers found a group of people they suspect of taking over a vulnerable person’s home to use it for drug dealing – a practice known as cuckooing. They said drugs were thrown from the building, four people were arrested and cash and assets seized.

Victoria Atkins, the minister for crime, safeguarding and vulnerability, said: “We are determined to crack down on county lines, disrupt the networks devastating communities and put an end to the violence and exploitation of children and vulnerable adults.”

Signs that a young person may have fallen prey to a county lines gang are said to be suddenly having new unaffordable belongings, going missing a lot, having friendships with older people or having unexplained injuries.

Every police force in England and Wales is affected by the gangs’ activities, the NCA believes, while the number of cases of modern slavery involving UK minors went from 676 in 2017 to 1,421 in 2018.

The operation was run by the National County Lines Coordination Centre (NCLCC), which is jointly run by the NCA and the National Police Chiefs’ Council.