Migrants who arrived in the UK after crossing the English Channel have been detained under the new “one in, one out” deal.
The first detention came as people arrived in Dover on Wednesday, the first day the pilot scheme came into force.
Pictures showed the migrants wearing life jackets disembarking from Border Force boats.
The Home Office said detentions began for those who arrived on Wednesday afternoon and they will be held in immigration removal centres until they are returned to France.
The first groups of people to cross the Channel were detained after their arrival at Western Jet Foil and will now be held in detention until they can be returned to France.
Home Secretary Yvette Cooper said: “Yesterday, under the terms of this groundbreaking new treaty.
“That sends a message to every migrant currently thinking of paying organised crime gangs to go to the UK that they will be risking their lives.
“They are also throwing away their money if they get into a small boat.”
UK officials aim to make referrals for returns to France within three days of a migrant’s arrival by small boat while French authorities will respond within 14 days.
This is in exchange for an approved asylum seeker in France to be brought to the UK under a safe route.
This process has now also been launched, with adults and families in France able to express an interest in coming to the UK through an online platform set up by the Home Office.
They will have to meet suitability criteria, standard visa application process and security checks.
If accepted, they would be given three months in the UK to claim asylum or apply for a visa.
It would be subject to the same rules for all asylum seekers not allowed to work, study or have access to benefits.
The Home Office is expected to launch a campaign in the coming days to make migrants in northern France and elsewhere aware of the new treaty.
Cooper added: “Criminal gangs have spent seven years embedding themselves along our border and it will take time to unravel them.
“But these detentions are an important step towards undermining their business model and unraveling the false promises they make.”
The Home Secretary has previously said the accord is not a “silver bullet” to stop small boat crossings.
It marked a step change as migrants would be sent back across the Channel for the first time.
Ministers have rejected criticism that the returns deal leaves open a loophole for human rights laws to be exploited for migrants to avoid deportation.
Efforts to crack down on illegal working and remove migrants with no right to be in the UK from the country are continuing as ministers grapple to curb the crossings.
