Greens will “beat the rising tide of far-right hate” across Europe, the party’s co-leader Jonathan Bartley will say as he opens the party’s conference.
The four-day event in Scarborough comes after the Green Party had its best European elections since 1989.
It was part of a broader rise in support for Greens across EU states.
Joint leader Sian Berry will tell the conference the major parties have “given ground to the right” and take aim at the Lib Dems over austerity.
“The Green Party are at the forefront of standing up to the far right – right across Europe,” Mr Bartley will say.
“They have their own violent vision for the future. But we hold the tools to stop them.”
He will say those considering joining the Green Party must do so now, adding: “We will beat the climate crisis and we will beat the rising tide of far-right hate.”
Although the Brexit Party won the most seats in the UK in May’s European elections, staunchly anti-Brexit parties the Lib Dems and the Greens also made gains.
Overall the Green Party came in fourth place, winning 12.1% of the vote and securing seven seats.
Many EU member states – from the Nordic countries to Portugal – also saw a rise in their Green vote.
Ms Berry will tell the Green Party conference: “The old politics is not working, and all the old parties are responsible.
“They have all given ground to the right, on freedom of movement, on Europe, on public spending.
“Labour and Conservatives yes, but let’s not forget that while the Lib Dems paint themselves as the defenders of liberal, internationalist values, they were all too happy to sign up to the austerity programme that has cost an estimated 130,000 lives.”
In Europe, there was a mixed result for the nationalist right, which had been expected to make significant gains.
Matteo Salvini’s right-wing nationalist League party won in Italy and Marine Le Pen’s National Rally party won in France.
But the nationalist parties did not do as well as anticipated in Germany or the Netherlands.