Nationwide operations began this week to detain adult men and women, with more activity due to be carried out over the next 11 weeks leading up to a one-way flight to east Africa.

Officials refused to say how many people had been held so far, but sources said there had been “dozens” of detentions across the UK, in cities including Glasgow, Liverpool, Birmingham and Bristol. Enforcement action is said to have taken place in England, Wales, Scotland and Northern Ireland.

In a further development, the senior civil servants’ union has submitted an application for a judicial review against the government’s Rwanda plan.

The FDA has said its members could be in violation of the civil service code if they follow a minister’s demands to ignore an urgent injunction from Strasbourg banning a deportation.

The Guardian disclosed on Sunday that detentions would begin on Monday. Several asylum seekers who turned up for routine Home Office appointments on Monday were detained and told that they would be sent to Rwanda.

The timing of the announcement on the Rwanda scheme, which is estimated to be costing more than £500m over five years, has prompted scorn from Labour. A party source said: “Is there any more blatant sign that [former immigration minister Robert] Jenrick was right about this all being symbolic before an election than this mad flurry of stories?”