The four men and three women - one with a young child - had taken part in a mass invasion of the construction site earlier this month, despite being named in a High Court injunction issued two days earlier.
Some wept and hugged each other as Mr Justice Alliott jailed them for 28 days for contempt of court. But he promised to consider releasing them early if they returned to the court on Monday to purge their contempt by swearing to abide by the injunction.
The seven gave passionate speeches saying it would be morally wrong not to protest against the Department of Transport's destruction of countryside rich in wildlife, landscape and archaeological value.
Fellow protesters vowed that the Department of Transport's legal action would only fan the flames of protest against environmentally damaging road schemes around Britain.
Paul Rogers, one of the 54 names on the injunction, who calls himself a green anarchist, said: 'The roads lobby is only making things worse for itself. If open protest is going to be squashed then you can bet your life people will resort to clandestine actions which can be highly effective.'
He was one of 13 ordered to appear at the High Court in London for allegedly breaking the injunction obtained by the Department of Transport. He told the judge he was not aware it was in force and so escaped prison. The other five did not appear and will be dealt with later; some have yet to be traced by the department's hired detective agency.
Emma Must, 27, a children's librarian, was too upset to make her statement so her co-defendant, Bob Bear, read it.
'I cannot stand idly by whilst the Department of Transport bulldozes through our countryside,' she wrote. 'Twyford Down is now a gaping chasm and the Itchen Valley at its foot has an enormous embankment stretching across it.
'I cannot consent to the wilful annihilation of such an immense swathe of our heritage by an ephemeral government.'
Ms Must's mother, Libby, said: 'I'm very proud of my daughter. She has been totally law abiding all of her life . . . she warned me this might happen.'
Mr Justice Alliott told the seven he had to defend the rule of law. 'Nothing is more saddening than when a judge is faced with the inevitable task of passing prison sentences on people who are fundamentally decent and motivated by a concern which to them overrides everything else.'
But he took a dim view of them violating the terms of their injunction within 48 hours of it being issued. 'You have been quick to snatch the martyr's crown and you may find it is uncomfortable head-gear,' he said.
The court viewed video recordings of BBC and ITN news bulletins on the day of the M3 invasion in which some defendants explained why they were breaking the injunction. Counsel for the Department of Transport also submitted newspaper reports and video recordings of the protest made by Bray's Detective Agency.
The seven jailed yesterday are Jason Torrance, Philip Pritchard, Simon Fairlie, Bob Bear, Rebecca Lush, Emma Must and Raga Fothergill, who has a 14-year-old son.