The environmental group plans to shut down key sites, including Westminster and Lambeth bridges, in addition to protests outside key Government departments.
The “opening ceremony” was held at Marble Arch on Sunday evening, featuring meditation and dancing as “inspiration” prior to the protests.
Groups of artists held a procession around Marble Arch as the protesters were told to “surround” the upcoming demonstrations with love.
Extinction Rebellion spokesperson Zoe Jones, 24, said Marble Arch will be used throughout the two-week protests as other sites are moved between.
She said: “We’re here to pressure the Government into action because we can’t wait any longer.
“The next two weeks will involve marches and family friendly events, there’ll be some spicier actions as well and some will be arrestable.
“We’ve had 4,000 rebels sign up and say they are willing to be arrested – which is a huge increase on the number arrested in April of 1,000.
“The public perception of XR is that we’re disrupting ordinary people’s lives by blocking roads and that’s why this time we’re taking our protests to the seat of power and taking it to Westminster.
“We are on the public’s side and we are ordinary people who are extremely concerned.”
Grace Maddrell, 14, told PA : “I’m here today because I am angry that no one is doing anything to save my future.
“I’m scared for that future and I’m here out of love for life and the animals that have gone extinct and everyone’s voice that has been silenced because they were not heard in time.”
“People are rebelling in these numbers because they realise the time to address this is right now, not in the year 2050, or even 2025. Scientists are despairing because we are almost out of time. They are telling us to panic.
“The weather won’t wait for politics, and so we will gather in Westminster and stay until the Government tells the truth that families across this country need to know.
“This includes the terrifying likelihood that our supermarkets will fail us because the food system is too fragile to continually withstand the shocks of extreme weather happening in other parts of the world.”
Many protesters attended Sunday’s opening with tents and supplies in preparation for the upcoming fortnight.
The group is expecting several thousand people to target different government departments, Downing Street and the Houses of Parliament over the next two weeks, according to a Extinction Rebellion spokesman.
Chay, a 23-year-old student from Bristol, said: “We’re here to hold our government to account for the shameful disregard for the climate crisis and wider environmental issues we’re facing at the current time.
“We have many, many actions going on over the next two weeks, I won’t give too much away as I want to leave something to the imagination but we are going to be focusing heavily on Westminster because we feel that our elected officials have let us down time and time again and we think it’s time they learned that we are here for the long run.”
On Saturday the group criticised the Metropolitan Police for alleged pre-emptive arrests and the confiscation of equipment including gazebos and beanbags.
Sunday’s protest was policed by two vans of officers who were stationed by Marble Arch, a small distance from the main protesters.
Ms Jones added: “There is a misconception with Extinction Rebellion that to be involved you have to be arrested but that is not the case.”
She continued: “At the moment we know we have 11 years to sort out our greenhouse emissions and that means in the next 18 months we have to have radical political change and the one way we have seen in history to get radical political change is to do non-violent political action.”
The protesters ranged in age from small children to the elderly, with many referencing the urgency of climate change for young people.
Grace added: “If you look at the people who are in government most of them are a lot older and they’re not going to be alive when this happens and they should be listening to us because they don’t have to worry about it but I do and it’s not fair.”
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