Green MP Caroline Lucas defended her decision to join campaigners at an anti-fracking protest, telling a court: "It felt important to serve the right to peaceful protest."
The Brighton Pavilion MP was arrested after linking arms with protesters outside energy firm Cuadrilla's exploratory drilling site in Balcombe.
She is on trial with four co-defendants who all deny obstructing a public highway and breaching an order under Section 14 of the Public Order Act last August 19.
Lucas, 53, told Brighton Magistrates' Court that she felt it was important to protest about fracking in a bid to prevent the UK being locked into using more fossil fuels.
She said: "I'm haunted by the idea that my children and my children's children will turn round to me and say, 'What did you do about this overwhelming threat?'
"And I want to do all I can do peacefully to address that before it's too late." Lucas said governments were failing to urgently address the issues surrounding climate change.
And she said she felt the protest outside Cuadrilla's main entrance gate was a "legitimate" and "appropriate" way of sending a message to Government about fracking concerns.
Lucas said she attended Balcombe the day before her arrest to take part in a march as the community was "at the frontline of this whole new fossil fuel industry".
The following day she joined protesters outside the main gate of the Cuadrilla site, which she said was not operating on those days.
She said it felt important and "symbolic" to be based outside the gate during the protest, which lasted around five hours from around 10.15am.
Lucas added: "If this protest hadn't been effective it would have been made harder to persuade the Government to stop fracking and go down cleaner energy routes."
Police approached her, pointing out a Section 14 notice under the Public Order Act had been issued, but Lucas denied being told of a designated protest area elsewhere.
She said: "I couldn't see any reason why we couldn't continue our peaceful protest."
Lucas said discussions were being held among the protesters at the time about wrapping up the protest before officers moved in.
The court heard that her son, who was sat next to her in the circle of protesters who had linked hands, was held by police.
"I was aware that the police tactics looked disproportionate," Lucas said. "I was aware that my son was in pain and I was very upset about it."
Pc Robert Staplehurst, of Sussex Police, approached Lucas, told her there was a Section 14 notice and asked whether there was anything he could say to move her, the court heard.
Lucas - who was elected as the Green Party's first MP in the 2010 General Election - replied no and she was then arrested.
She went on: "If we had another 10 to 15 minutes we could have come to an agreement to end it and dispersed. We weren't doing anything different than we were in the previous five hours."
Lucas, from Brighton; Josef Dobraszczyk, 22, from Bristol; Ruth Jarman, 50, from Hook, Hampshire; Sheila Menon, 42, from north east London; and Ruth Potts, 39, from Totnes, Devon, all deny wilful obstruction of the highway and breaching Section 14 of the Public Order Act.