Video report on Extinction Rebellion by ITV correspondent Rebecca Barry
Climate change protesters have revealed they plan to "shut down Heathrow" on Good Friday and disrupt the Easter holiday plans of thousands of travellers.
However Scotland Yard has warned the protesters face a "robust response" if they target Heathrow and added it has ''strong plans'' in place.
Heathrow added it is "working with the authorities" to address any threat of mass disruption, while Home Secretary Sajid Javid urged police to use the "full force of the law".
Eco-activists from Extinction Rebellion (XR) have been causing large-scale disruption in London since Monday as they campaign against climate change.
Some 500 people have been arrested since Monday, 10 have been charged - with three for gluing themselves to a train.
Police have been criticised for failing to clampdown on XR, with many commuters voicing anger as the protests look set to enter a fifth day.
A 1,000 officers have been deployed to demonstrations in the capital each day.
Metropolitan Police Assistant Commissioner Nick Ephgrave said the protests were "increasing in intensity" and warned demonstrators against targeting Heathrow Airport.
Around 500,000 people are expected to fly out of Heathrow over the bank holiday weekend.
In a statement, the Met added: "The airport is part of our national infrastructure and we will not allow the illegal activities of protesters to cause further disruption and misery to thousands of travellers, many of them families, over Easter.
"We would urge any protester planning to attend Heathrow to strongly reconsider."
Environmental protests continued into a fourth day on Thursday and saw activists issue a challenge to Environment Secretary Michael Gove to meet with them in Parliament Square to find a solution to the issue.
Roadblocks across the capital remained in place as illegal demonstrations continued at Waterloo Bridge, Oxford Circus and Parliament Square, while demonstrations are allowed to continue only at Marble Arch.
So far, more than 480 people have been arrested in connection with the protests, largely for obstructing the highway and breaching the Section 14 order that prohibits protests apart from at Marble Arch.
A total of eight people have been charged with those offences, while a further three people who have been charged by British Transport Police appeared in court on Thursday morning and were remanded in custody after denying obstructing a DLR train during a climate change protest.
On Thursday afternoon, more than 200 cyclists joined a mass protest against climate change in the heart of the capital.
ITV News Correspondent Penny Marshall talks through what has been happening on the fourth day of protest in central London
Sajid Javid said more than 1,000 police officers a day have been deployed to deal with the demonstrations in the capital as he urged police to use the "full force of the law".
"They [activists] have no right to cause misery for the millions of people who are trying to lead their daily lives," the Home Secretary said.
"Unlawful behaviour will not be tolerated."
The Met has said the protests are putting a strain on police in the capital and diverting officers away from their "core local duties".
Officers in the force have been asked to work 12-hour shifts, while rest days have been cancelled.
The force said that although the Violent Crime Task Force (VCTF) has been ring-fenced, the protests are diverting officers away from their local duties and those within the team have had their leave cancelled.
London Mayor Sadiq Khan has written a letter to the Home Secretary about the increasing cost of policing a growing number of protests in London, and he is working with the Met to consider making a special grant claim over the current demonstrations.
Yet protesters remain defiant, telling ITV News a lack of listening from world governments over the last three decades has led to them having to take such drastic action.
Protesters explain why they're camping out in central London
But the tactics used buy some of the protesters have caused anger, both from commuters and the authorities.
However, organisers have said they expect even more people to join the protests over the bank holiday weekend, with a statement - which was later deleted - reading: "Easter weekend is tomorrow and thousands more rebels will join.
"Police struggle to arrest 350 and there are 10 times that number prepared to be arrested.
"The hollowed-out British state is overwhelmed."
Speaking from Waterloo Bridge, Dr Gail Bradbrook, a co-founder of the XR group, said demonstrators would continue to act despite the first people being charged over the disruption.
"It (the charges) might put some people off and we escalated our strategy by focusing on the rail infrastructure," she said.
"It's certainly an option that tactics will be escalated if our demands are not met," she said.
Some Londoners became annoyed with protesters on Thursday
Activists say they plan to continue roadblocks, which have affected more than half a million people with traffic gridlock and disruption to transport and businesses since Monday, until at least next Friday.
Speaking on Wednesday, Environment Secretary Michael Gove said that while it was "appropriate for people to make their feelings known, I also think, we've got the message, we understand that action needs to be taken".