Environmental protesters plan to target London's Heathrow Airport as hundreds of thousands of passengers fly out over the Easter weekend.
The airport said it was "working with the authorities" to address the threat of mass disruption, while UK Home Secretary Sajid Javid has urged police to use the "full force of the law".
Mr Javid condemned activists who are breaking the law as organisers threatened to escalate the group's tactics if their demands are not met.
Scotland Yard has arrested more than 500 protesters amid ongoing Extinction Rebellion (XR) demonstrations across four locations in central London, comprising Waterloo Bridge, Marble Arch, Oxford Circus and Parliament Square.
Three demonstrators were remanded in custody after they were charged over their alleged involvement in obstructing trains at Canary Wharf station yesterday morning.
All three indicated a not guilty plea when they appeared at Highbury Corner Magistrates' Court.
A spokesman for the XR group said activists will be staging protest action at Heathrow Airport tomorrow, with around 500,000 people expected to fly out for Easter breaks over the bank holiday weekend.
A Heathrow spokesman said: "We are working with the authorities to address any threat of protests which could disrupt the airport."
Speaking after a briefing with Metropolitan Police Commissioner Cressida Dick today, Mr Javid said she had told him that more than 1,000 officers were being deployed to the streets of the capital each day of the protests.
He said: "Over recent days, commuters trying to earn a living have been unable to travel to work and businesses have been disrupted.
"Emergency vehicles have faced difficulties navigating the road networks and the demonstrations have put added pressure on police officers whose job it is to solve crimes and protect the public.
"Let me be clear - 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."
Let me be clear - 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. (4/6)
— Sajid Javid (@sajidjavid) April 18, 2019
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 is working with police to consider making a special grant claim over the current demonstrations.
The fourth day of protests, which have seen activists glue themselves to public transport and block bridges and major roads across the capital, began with a challenge to Environment Secretary Michael Gove to meet activists at Parliament Square to find a solution to the issue.
Organisers said they expected even more people to join the protests, with a statement - which was later deleted - adding: "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's certainly an option that tactics will be escalated if our demands are not met," she said.
