Almost 100 branches of Barclays bank were unable to open for business on Monday after Greenpeace obstructed the entrances.
The campaign targeted branches across the UK to protest against the bank's funding of oil and gas companies.
Greenpeace claims among banks, Barclays is the biggest funder of fossil fuels in Europe. It wants the bank to switch its funding into renewable energy.
Barclays said it is working to get the branches open as quickly as possible.
In the early hours of Monday morning, 97 Barclays branches were targeted by Greenpeace, which disabled the doors preventing staff from entering.
Images of people bearing slogans such as "Stop Funding Fossil Fuels" were stuck on the windows and "pop-up exhibitions" displaying photographs of climate change were used to block major Barclays branches in Belfast, Cardiff, Edinburgh, London and Manchester.
"Barclays must stop funding the climate emergency, that's why we've taken action today. From floods to bushfires and record heat in Antarctica, the impacts of this crisis are staring us in the face," said Morten Thaysen, climate finance campaigner at Greenpeace UK.
"Yet Barclays keeps pumping billions into fossil fuel companies at exactly the time we need to stop backing these polluting businesses," he said. "Banks are just as responsible for the climate emergency as the fossil fuel companies they fund, yet they've escaped scrutiny for years. We've shut down branches across the country to shine a spotlight on Barclays' role in bankrolling this emergency. It's time Barclays pulled the plug and backed away from funding fossil fuels for good."
A Barclays spokesperson said: "We recognise that climate change is one of the greatest challenges facing the world today, and are determined to do all we can to support the transition to a low carbon economy, while also ensuring that global energy needs continue to be met.
"Greenpeace has a view on these issues to which they are completely entitled, but we would ask that - in expressing that view - they stop short of behaviour which targets our customers, and our colleagues, going about their lives in communities around the country."
The bank is working to get the affected branches up and running but does not have an exact timescale for full service to resume.