"A number of protesters remain outside the front of the airport terminal and we are working hard with the Metropolitan Police to minimise disruption to passengers".
The group said it was targeting London City Airport because the airport was planning a $3.64 billion expansion.
"It is used disproportionately by private jets and by financiers, business people and other members of the polluter elite flying in and out of the City of London, when often they could do what they need just as well by using digital telecommunications".
Airport and Aer Lingus airline officials told AFP that a morning flight to Dublin had to be delayed by almost two hours after a ticketed passenger got up to make a speech about the climate as the plane prepared to take off.
The year-old group backed by Swedish teen activist Greta Thunberg is spearheading a two-week global action that kicked off Monday with partial shutdowns of traffic in cities stretching from Sydney to NY.
The Czech branch of the Extinction Rebellion environmental movement will block traffic and hold protest gatherings in Prague to draw attention to the climate crisis from Thursday, October 10 to Saturday, October 12, the movement informed on social media on Wednesday.
More than 500 people have been arrested in London since protesters began blocking roads and bridges on Monday, part of a global wave of protests that have also hit New York, Paris, Berlin and Madrid.
The busy transportation hub is the capital of the fifth-largest airport, popular with business travelers, bankers and politicians for short-haul and regional routes.
Police are now being more assertive.
"Should you fail to comply with the conditions you run the risk of being arrested and prosecuted".
"We are protesting against Government inaction and climate and ecological breakdown".
Britain has set itself the target of 2050.