The U.K. has started rounding up and detaining asylum seekers as part of its highly contested plan to deport migrants to Rwanda. Rights groups are hoping legal challenges can stop the deportation flights, which are expected to start as early as July. Prime Minister Rishi Sunak’s Conservative government overrode a Supreme Court ruling in order to ram through the policy.

On Thursday, London police arrested dozens of protesters who laid down on the road and slashed tires to stop a bus from carrying detainees to a barge that has been converted into a migrant prison. Authorities were forced to postpone the planned transfer of asylum seekers.