As climate change protests continued for a fourth day on Thursday, leaked WhatsApp messages suggested activists are planning to "shut down" Heathrow Airport on Good Friday.
The airport said it was "working with the authorities" to address any threat of disruption over the long Easter weekend.
It came after Metropolitan Police updated the arrest figure to 428 on Thursday.
Home secretary Sajid Javid offered his support to the police and condemned any "unlawful protests", while London mayor Sadiq Khan suggested cuts to police funding were restricting the Met's ability to cope with the demonstrations.
Three activists appeared in court where they pleaded not guilty to charges relating to their alleged involvement in obstructing trains at Canary Wharf station on Wednesday morning.
They were remanded in custody for a month until they next appear at Blackfriars Crown Court on 16 May.
Follow how the protests develop throughout the day
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