Over 200 demonstrators in the UK had signed up to join the international fast, which will see a week of hunger strikes in at least 27 countries, the group said.
The organisation said its activists “will be outside all UK political party HQs all week demanding leaders meet us and talk about our three demands bill”.
It also said it delivered letters to party leaders “demanding a one hour meeting on camera and a commitment to implement” the bill.
The three demands bill calls on the government to “tell the truth by declaring a climate and ecological emergency,” act to halt biodiversity loss and reduce greenhouse gas emissions to net zero by 2025, and be led by a citizens’ assembly on climate and ecological justice.
The group tweeted to say the Green Party had met with hunger strikers and agreed to a leaders meeting.
“Protests will continue round the clock outside other major parties HQ’s [sic],” it said.
In the letter Extinction Rebellion sent to party leaders, the group said: “Today, humanity is at a crossroads. Either we unite to prevent the rapidly escalating climate and ecological collapse, or this nation and global society will disintegrate beyond recognition.
“Your party has an absolute duty of care, for the current and future wellbeing of everyone in the United Kingdom.”
It concludes: “This request is being made by Extinction Rebellion to the seven main parties contesting the UK general election. The responses will be collated and published.
“Polls show that at this election the majority of voters will consider party responses to the emergency when deciding how to vote.”
A spokesperson for the Labour Party told The Independent: “The Labour party has declared a climate and environment emergency and is committed to working on a pathway towards net zero carbon emissions by 2030.
“Our manifesto will set out that path.”
The Independent has contacted the Conservative Party for comment.
Angus, 50, who is on hunger strike in London, said: “The future of my nephews, my niece, my godchildren, this country and humanity, are now critically vulnerable to the decisions being made by our politicians”.
La Pethick, 83, from the Brighton area, said: “I am going on hunger strike for my family’s future and for future generations.”
The protest has begun as Britain gears up for a general election on 12 December.
It came on the same day Labour dropped a radical plan to end the UK’s contributions to climate change by 2030, instead saying it would stick to a target of achieving it “well before 2050”.
Voters care more about the environment ahead of the coming election than at any time in the past, according to an Ipsos Mori poll, which put the issue ahead of the economy, education and immigration.
Extinction Rebellion won a victory earlier this month when the High Court ruled the Metropolitan Police’s London-wide ban on its protests was unlawful.
Nine cases against Extinction Rebellion, relating to public order offences, were suspended in the wake of the legal challenge.