Extinction Rebellion and other campaign groups have vowed to “step up” action unless the government responds to two demands on climate change.
XR has called on ministers to end all licences, funding and approval for new oil and gas projects.
The group also wants the government to create “emergency citizens assemblies” to tackle the climate crisis.
XR said it had given Downing Street a deadline of 5pm on Monday, 24 April to reply to the demands or it would escalate its actions.
It comes ahead of four days of demonstrations taking place in Parliament Square, London, beginning on Friday, 21 April.
More than 200 organisations are supporting the protests – including Greenpeace and Friends of the Earth, XR said.
Some 28,000 people have so far said they will attend the protest, although XR expects more to join the demonstrations over the course of the weekend.
In a joint press conference on Tuesday, XR insisted that the demonstrations were not intended as “public disruption” but accepted that the sheer number of people attending may cause “logistical issues” in and around Westminster, with the London Marathon taking place on Sunday.
XR has previously called on the government to end new licences for fossil fuel projects but its demands have been so far ignored.
If ministers do not respond by the stated deadline, XR said escalated protests would involve joining picket lines “in solidarity” with workers who are on strike.
“We must unite to survive,” said Marjin van der Geer from XR.
This weekend’s protest is being billed by XR as “The Big One” and will be attended by a range of climate, environment and poverty campaign groups.
The Public and Commercial Services (PCS) Union, which mainly represents civil servants working in government departments, will be joining the protest.
Last week civil servants vowed to take further industrial action after rejecting what it described as the government’s “insulting” 4.5 per cent pay rise in an ongoing dispute over pay.
The UK has witnessed a series of high-profile protests by climate and animal rights protesters in recent days.
On Saturday the Grand National horse race in Liverpool, was delayed after large numbers of protesters tried to storm the track.
Scores of activists climbed fences at Aintree, with at least two fixing themselves to a jump using glue and lock-on devices, animal rights group Animal Rising said.
The protesters breached security fences as National runners were in the parade ring, causing a delay of 12 minutes, although racegoers helped police and event organisers to stop some from reaching the track.
Merseyside Police said officers made 118 arrests.
On Monday night play was suspended at the World Snooker Championship after a Just Stop Oil protester ran into the arena, jumped onto the table and scattered a bag of orange powder across the surface.
The incident happened during Robert Milkins’ clash with Joe Perry in the evening session, forcing a delay of around 40 minutes before any play resumed.
Earlier this year XR said it would “temporarily” stop using public disruption tactics in 2023 after a year of protests that caused chaos on Britain’s roads, sparking angry clashes with motorists.
The group said it would continue to protest against the “abuse of power” by vested interests who it said were “addicted to greed and bloated on profits” but would stop using public disruption as a “primary tactic”.
*This article was amended at 10:21 on 19 April. A previous version incorrectly said XR had given the government a deadline of Tuesday, 25 April to reply to its demands. Don’t Pay UK and the PCS Union have not yet signed up to XR’s two demands, as the previous version reported.
