Heathrow Airport is braced for disruption after climate change protesters announced plans to target the transport hub.
Home Secretary Sajid Javid has urged police to use the “full force of the law” to deal with illegal Extinction Rebellion (XR) demonstrations as they enter a fifth day.
More than 500 protesters had been arrested by Thursday night as activists continued to ignore orders to leave roadblocks at Waterloo Bridge, Oxford Circus and Parliament Square.
Organiser said action would be escalated to include Britain’s busiest airport on Good Friday, with around 500,000 people expected to fly out for Easter breaks over the bank holiday weekend.
Scotland Yard has warned protesters the force had “strong plans” in place with a significant number of officers ready to respond.
The airport said it is “working with the authorities”, while Metropolitan Police Assistant Commissioner Nick Ephgrave said: “Protesters can expect a robust police response. We are determined to keep the airport operating.”
Mayor of London Sadiq Khan said he was “extremely concerned” about the possible disruption.
“This is extremely dangerous, illegal and is putting an unacceptable toll on our police force and our city,” he said.
More than 1,000 officers have been deployed daily to deal with the protest.
Some have been asked to work 12-hour shifts, while rest days and leave has been cancelled.
The Met said the protests are putting a strain on policing in the capital with officers diverted away from their “core local duties”.
After a briefing from Met Commissioner Cressida Dick, the Home Secretary said: “I totally condemn any protesters who are stepping outside the boundaries of the law”.
“They have no right to cause misery for the millions of people who are trying to lead their daily lives. Unlawful behaviour will not be tolerated.
“I expect the police to take a firm stance and use the full force of the law. They have my full backing in doing so.”
Mass arrests for obstructing the highway and breaching the Section 14 order that prohibits protests apart from at Marble Arch do not appear to have deterred protesters and so far 10 people have been charged with those offences.
A further three people who were charged by British Transport Police appeared in court on Thursday morning.
Cathy Eastburn, 51, Mark Ovland, 35, and Luke Watson, 29, were remanded in custody over their alleged involvement in obstructing trains at Canary Wharf station on Wednesday morning.
Organisers said they expected even more people to join the protests and warned they would continue until their demands are met.
The group wants the Government to declare a climate emergency and take action to reduce greenhouse gas emissions to net zero by 2025.
Actress Dame Emma Thompson joined the protests in London after flying from Los Angeles.
In a video shared by the campaign group on Thursday, the Hollywood star urged viewers to “come and join” the demonstrations.
A representative of Dame Emma said she needed to take the 5,400-mile flight home to London after working in LA.
Large numbers of protesters moved from Oxford Circus to Parliament Square on Thursday night after organisers announced police were planning to make arrests in Westminster.
Sir David Attenborough’s BBC documentary Climate Change: The Facts was projected onto Westminster Abbey.