Campaigners have labelled the jailing of a Hampshire barrister who refused to pay child support as "scandalous".
Michael Cox, 43, argued that as his three children lived with him half of the time, he should not have to pay.
But Southampton magistrates jailed him for 42 days for not paying £43,000 owed to the Child Support Agency, despite his ex-wife supporting Cox in court.
Now Fathers 4 Justice - which promotes the rights of fathers - is planning a "Free Michael Cox" campaign.
The barrister, from Hythe, near Southampton, was given a suspended 42-day jail sentence in March, but continued to withhold payment.
On Monday he was brought before magistrates having amassed debts of £43,000 to the Child Support Agency (CSA) since he separated from his ex-wife Lesley Peach in 1994.
Ms Peach, 39, who the CSA said should have received £365 a month, begged magistrates not to jail him, arguing that she would not be able to work without Cox looking after their children.
Matt O'Connor from Fathers 4 Justice called Cox a "Guantanamo dad" and a "political prisoner".
"Prisons are bursting at the seams and yet here is a man that has been vindictively pursued by the CSA," said Mr O'Connor.
"To jail him on a matter of principle is scandalous.
"He already looks after, feeds, clothes and provides a roof over their heads half the time. Why should he have to pay that money twice?
"He's treated as an absent father because of his gender."
'Scapegoat'
Cox, a legal adviser to Fathers 4 Justice, has since remarried and has two young children with Beth Cox, 32, a primary school teacher.
They live one mile away from his ex-wife and their three children.
Mr O'Connor added: "We think he is a scapegoat because he has a role in our organisation."
The Department for Work and Pensions (DWP), which governs the soon-to-be-disbanded CSA, said it was "determined to get more money to more children".
"We always give non-resident parents every opportunity to comply with their responsibilities, but will take enforcement action where necessary," it said in a statement.
"We will continue to reform the system of child maintenance to ensure that all parents fulfil their duty of care."