Zack Polanski said two-party politics is "dead and buried" as the Green Party celebrated local election gains, including its first-ever elected mayors.
The Green Party of England and Wales leader said his belief they could replace Labour was being backed up by results at the ballot box.
They took control of Norwich, Hastings and the London borough of Waltham Forest, as well as winning the mayoral contests in Hackney and Lewisham.
The party saw its first two members elected to the Welsh Parliament while the Scottish Greens also won their first-ever constituencies at the Scottish Parliament.
Polanski said some people were voting for the Greens as they were "disillusioned" with Labour, but he added it was exciting that "lots of people" were supporting the party for the first time.
The increased support for the Greens paved the way for the party gaining seats in different parts of England, including Manchester, Sheffield, Leeds, Oxford and Exeter.
One of the party's notable victories came in Hackney as Zoë Garbett defeated her Labour opponent to win the mayoralty.
Polanski described it as a "historic victory" and it was followed by Liam Shrivastava's win over Labour to become mayor of Lewisham.
Speaking after the Hackney mayoralty victory, Polanski said: "Two-party politics is not just dying, it is dead and it is buried.
"And actually, whether it's here that Labour have been rejected, or whether we're seeing around the country, it's very clear that the new politics is the Green Party versus Reform."
On whether it was a positive vote for the Greens or a protest vote against the Labour government, Polanski said there had been a "rise in the Green vote".
He added: "I think people are both sick of Labour but also really excited by a Green alternative."
Polanski said he believes Prime Minister Sir Keir Starmer "needs to go", saying: "But I don't think that's my message, I think that's the country's message."
Green Party deputy leader Rachel Millward said her party had "massively increased our vote share pretty much everywhere we've stood", including a "massive breakthrough" in London and gains in Manchester that were "over our expectations".
Polling expert Sir John Curtice said the Greens had recorded their "best-ever performance", as he revealed the projected national share of the vote for Britain.
This showed the Greens on 18%, behind Reform UK on 26% but ahead of Labour and the Tories, who were both on 17% and the Liberal Democrats on 16%.
This was on the basis of results in more than 1,000 wards where the BBC collected detailed voting data, assuming that people who did not have an election voted similarly to those who did.
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