The British authorities on Wednesday started to detain migrants ahead of their deportation to Rwanda in the next nine to 11 weeks.

The latest moves is obviously laying the groundwork for Prime Minister Rishi Sunak’s flagship immigration policy.

A law that paved way for asylum seekers who arrived Britain without permission to be sent to Rwanda was approved by parliament in April, and Sunak wants the first flights to take off in July.

More than 7,500 migrants have arrived in England on small boats from France so far this year, and the government says the policy will deter people from making dangerous journey across the Channel.

Human rights charities and unions opposed to the policy are expected to launch fresh legal challenges to stop the flights from taking off after the UK Supreme Court declared the policy unlawful last year.

Reuters reports that images released by Britain’s interior ministry on Wednesday showed a man being put in a van by immigration enforcement officials, and another being led out of his house in handcuffs.

“Our dedicated enforcement teams are working at pace to swiftly detain those who have no right to be here so we can get flights off the ground,” interior minister James Cleverly said in a statement on Wednesday.

Care4Calais, a refugee charity, said the detentions had started on Monday.

A spokesperson said that the group’s helpline had received calls from “tens of people”, adding that they still did not know who would be earmarked for the first deportation flight, or when it would be attempted.

Britain sent its first asylum seeker to Rwanda under a voluntary scheme, The Sun Newspaper reported on Tuesday, a separate programme to the deportation policy.

“People are very frightened,” said Natasha Tsangarides, Associate Director of Advocacy at charity Freedom from Torture, saying the fear of being detained and sent to Rwanda would push some people to go underground and disengage with their support system.