But despite the action at London City Airport, which included some people gluing themselves to the floor, flights appeared to be unaffected.

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The activists are attempting a three-day "Hong Kong-style occupation of the terminal building" to highlight what they claim is the "incompatibility" of the east London airport's planned £2 billion expansion with meeting the Government's legally binding commitment to go net carbon neutral by 2050.

Police made several arrests as the group sat and chanted "Fly today, gone tomorrow" in unison, as members of the public struggled to get past with their suitcases.

Those arriving for flights were redirected to a second terminal entrance by security workers and were not allowed to enter the building without showing their boarding cards first.

An activist who gave her name as Claire, 51, told the PA news agency: "I don't know what's going to happen over the next few hours, but I do know that a number of people have come here, City Airport, today to make the statement that there are many areas of our lives that are going to have to change because of the climate crisis we've created, and one of them is flying.

"We can't carry on with life, with business as usual," she said.

Claire said she had not made up her mind about whether she would allow herself to be arrested, but was later led away by police officers.

The sit-in comes on the fourth day of the Extinction Rebellion climate change protests, which are taking place across central London and other major cities worldwide.

A spokesman for the airport said: "We can confirm that a number of protesters have arrived at London City Airport.

"We continue to work closely with the Metropolitan Police to ensure the safe operation of the airport, which remains fully open and operational.

"As of 9.15am, there have been over 60 flights which have either arrived or departed."

In Westminster, tents and protesters had been cleared away from the roads leading into Parliament Square.

But the streets around Parliament and Whitehall remained closed to traffic, apart from cyclists, amid a heavy police presence in the area.

At Trafalgar Square, tents, banners with messages such as "This is change" and a hearse still blocked the roads, and a protest camp occupied the pedestrian area of the square.

More protesters were camped with dozens of tents in nearby St James's Park.

On Wednesday night, rebels continued their protests in Trafalgar Square.

They were joined by Benedict Cumberbatch and Simon Amstell, while electronic music duo Disclosure played a set.

Extinction Rebellion activists glue themselves to floor at London's City Airport