Several hundred supporters of the three protesters jailed for blocking access to the Preston New Road fracking site in Lancashire have gathered outside the high court in London before their appeal against their sentences.

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Simon Blevins, 26, and Richard Roberts, 36, were jailed for 16 months, and Rich Loizou, 31, was jailed for 15 months, after a four-week trial last month led to their convictions for causing a public nuisance.

They were granted an expedited appeal amid widespread anger at punishments that were held to be “excessive and extraordinary” for peaceful protesters. Liberty, the human rights organisation, has also intervened in their case, which it warned could have a chilling effect on future protests.

Supporters carrying roses and holding banners gathered outside the Royal Courts of Justice on the Strand from 8am. Caroline Lucas, the Green party MP, was among those who addressed the crowd, calling the decision to frack in Lancashire “an absolute trampling over democratic rights” .

She said: “We are here to uphold the right of peaceful protest. We are here to celebrate those people who are brave enough to put their bodies where their beliefs are. Friends, that is a powerful thing to do, and that is why our government wants to criminalise it. That is why three very brave people are on trial today.

“But we are here to say that the protest goes on. That whatever the outcome in the Royal Courts of Justice today, our battle goes on, because we have right on our side. We know that when people look back on this time it will be those protesters inside and all of us outside who will be shown to be on the right side of history.”

Sharron Loizou, whose son, Rich, was jailed, said she was hopeful about the outcome of the appeal. But her husband, Platon, said: “We are also in a situation where this is GB Green Week. In GB Green Week the government is allowing Cuadrilla to start fracking, the government is doing its utmost to support this dreadful polluting industry and it should stop.”

Sharron Loizou added: “This is about being able to peacefully demonstrate and I think that’s really important democracy. To imprison people for being peaceful demonstrators is awful.”

The partner of Richard Roberts, whose name was given only as Shelley, told the protest that on a visit to Preston prison the defendants had told loved ones they were collecting pictures sent in by supporters and sticking them on the walls of their cells with toothpaste.

“They keep falling down, but that’s OK because they’re actually running out of room [because] they’ve had so many postcards and beautiful letters from all over the world,” she said. “They’re actually doing OK in there, they are being treated very well. They are all together, they are on the same wing, so they are feeling quite strong. And they have absolutely loved this week, seeing all of the actions at Preston New Road.

“I think if there’s one thing that they would really love, [it] is for everybody who possibly can, and who is physically able, to go on Saturday to Preston New Road.”

Sir Ian Burnett, the lord chief justice, is leading a panel of three judges hearing the appeal, which was due to start at 10.30am.