Almost 400 people were arrested on the seventh day of protests on the A12 motorway by environmental campaigners on Friday, including a young boy and his mother.

The demonstration began at around midday when the first protestors walked onto the road, closing it in both directions. After a time, police again used water canon to disperse the crowd.

Friday’s protest was bigger than those held earlier this week. Supporters of Extinction Rebellion have been blocking the motorway every day since last Saturday when several thousand people turned up and 2,400 were arrested.

They have pledged to continue their protest until subsidies for fossil fuels are stopped.

Police unions are now calling on the government to intervene, saying the protests are taking up too much police manpower and stopping them from doing other work.

A second demonstration involving around 100 school pupils calling for action on the environment took place on the nearby Malieveld on Friday, and was sanctioned by the city council, the AD reported.

Several minors were among those arrested on Saturday and it later emerged that their parents had been reported to social workers at the Veilig Thuis organisation. A spokesman told news agency ANP on Friday that no new cases had been reported to them since the weekend.

Veilig Thuis had said it was “surprised” to have received dozens of referrals about parents who took their children to the demonstrations on Saturday and Sunday.