Retired teacher Gaie Delap, from Bristol, said the “climate emergency” was not being taken seriously enough and told Mr Justice Soole on Tuesday that her “heart was breaking” for the future of “my six grandchildren”.
Ms Delap, a mother of two, said she had been given climbing training by a Just Stop Oil “mentor” before taking part in the protest on the M25 in November last year.
She said a policeman had greeted her with “some surprise” after being called to deal with the protest and climbing on to the gantry.
Ms Delap is one of 12 Stop Oil protesters accused of breaching a court injunction aimed at restricting protest on the M25 and causing disruption.
Why did I do it? My heart is breaking for the future of my six grandchildren and all future generations
Mr Justice Soole is overseeing a hearing at the High Court in London which is expected to end later this week.
Lawyers representing National Highways say protesters caused “considerable delays” and were in contempt of court.
Ms Delap said she was unaware that an injunction was in place and apologised – other defendants said the same.
She told the judge she had joined the protest after Just Stop Oil called for volunteers.
“I heard a Just Stop Oil Zoom call for climbers to volunteer,” she said.
“I thought, ‘well, I am 76, I have never had any climbing experience’.
“I didn’t think I had to volunteer for that. But it went onto my mind and stayed there.
“I then followed it up with an inquiry about how I might get involved.”
Ms Delap, who said demonstrators had gone to a “safe house” the night before the protest, said she had climbed a ladder and sat on the gantry.
She said a policeman had arrived after a “long time”.
“A policeman climbed up and said ‘are you on your own?’ – in some surprise,” she said.
“I told him I was perfectly safe – I was attached to the gantry with a carabiner.”
Ms Delap added: “Why did I do it?
“My heart is breaking for the future of my six grandchildren and all future generations.”
She told the judge that Just Stop Oil wanted the Government to “stop new oil and gas” and went on: “The climate emergency is a serious issue and it has not been taken seriously enough.”
Ms Delap estimated, after the hearing, that the gantry was about five metres – more than 16 feet – high and said she had been on it for several hours.
Paul Sousek, 72, Theresa Norton, 65, and Paul Bleach, 56, also feature in the litigation.
Others protesters accused of contempt at the hearing in the Royal Courts of Justice, in London, are: Charlotte Kirin, 54; Daniel Johnson, 25; Joseph Linhart, 22; Luke Elson, 30; Mair Bain, 36; Paul Bell, 23; Rosemary Jackson, 25; and Theresa Higginson, 25.
:: Just Stop Oil said protesters had staged a demonstration at law firm DLA Piper. A spokeswoman said the firm was “responsible” for National Highway injunctions. She said three Just Stop Oil supporters had displayed banners and “deposited a large amount of legal paperwork” on to the floor.