The pair of demonstrators on top of the train at Canary Wharf station, in the heart of one of London's financial districts, also unfurled a banner that said "Climate Emergency".
The stunt comes on the third day of a series of climate change protests which have led to almost 300 people being arrested in the capital.
The group has blocked four main traffic arteries in central London since Monday, and on Wednesday it began a day of "light" disruption of the city's overground network.
Activists had been told they must confine any demonstrations to the Marble Arch area, and police were taking action against protesters in other locations.
"Targeting public transport in this way would only damage the cause of all of us who want to tackle climate change, as well as risking Londoners' safety and I'd implore anyone considering doing so to think again". Another activist glued himself to the side of the train.
Protestors block the roads in parliament square in London, on April 17, 2019, with Big Ben's clock tower shrouded in scaffolding back right.
The DLR is now only running minor delays, according to Transport for London. British Transport Police said they had arrested one man on suspicion of obstructing the railway.
In a statement posted on their website, Extinction Rebellion said they would "nonviolently disrupt Tube services to highlight the emergency of ecology collapse". "As with a labor strike, economic disruption is key in forcing the government to come to the table and negotiate our demands".
He added: "Of course, I am extremely frustrated by the damages causing to the public transport system in London, and that is why I would say to the organisers, please work with the police, please work with TfL to make sure you minimise disruption caused to those trying to get about their business in our city".
The protests have cost over 12 million pounds ($16-million) to businesses in London's West End, famous for its theatres and shops, with some seeing a 25 per cent drop in sales and footfall.
The Met warned that demonstrations are expected to continue "throughout the coming weeks". But she told BBC radio that "we need to take actions that are disruptive so everyone understands the dangers we're facing right now".
Extinction Rebellion, which generated headlines with a semi-nude protest in the House of Commons earlier this month, is demanding the government reduce greenhouse gas emissions to net zero by 2025.
The EU has finally adopted the Copyright Directive
For example, Germany said that it must be the aim to render so-called "upload filters" largely unnecessary in practice. The UK, Germany and France were among the countries that voted in favour of the reforms.