European leaders meeting at a summit in Brussels will make a new attempt to set the European Union on course for carbon neutrality by 2050, in a test of the bloc’s credibility on the climate emergency.

Hours before EU leaders were due to arrive on Thursday, Greenpeace activists unfurled a banner on the side of the summit venue warning of the climate emergency.

Activists in climbing gear scaled the Europa building and set off distress flares and alarms as the group released a statement accusing leaders of “letting [the world] burn”. Some 28 people climbed the building known as the space egg, according to the group.

Associated Press reported that 20 people on the ground were detained.

Most EU member states support the plan to make the EU carbon neutral by 2050 but Hungary, Poland and the Czech Republic are refusing to sign up, raising the prospect of a dramatic setback for Europe’s green credentials in the final days of the UN climate conference in Madrid.

Failure to agree the EU-wide target a second time, after a previous attempt in June, would be a blow to the European commission president, Ursula von der Leyen, who on Wednesday set out her vision for a green deal that would transform every aspect of economic life.

The green deal – a sweeping plan to change food production, industry, transport, buildings and energy use – is based on a target for net-zero carbon emissions by 2050. Von der Leyen wants a €100bn (£84bn) fund of public and private money to help countries abandon fossil fuels, in addition to a plan from the European Investment Bank to incentivise €1tn of green investment between 2021 and 2030.

Facebook Twitter Pinterest Ursula Von der Leyen has set out her vision for a green. Photograph: Piroschka Van De Wouw/Reuters

But EU member states are stuck over the sequencing of the 2050 target and money. The three central European countries want generous and precise promises of EU funds before they sign up to the target. France, Germany, the Netherlands and other net payers into the EU budget think the climate pledge comes first, as they do not wish to preempt discussions on the EU’s next seven-year budget, due to be concluded in 2020.

Q&A What is the European Green Deal? Show Hide The European Green Deal is the EU’s answer to what the European commission’s new president, Ursula von der Leyen, called the “existential issue” of the climate emergency. Most EU countries have signed up to goal of a climate neutral EU by 2050, a goal demanding dramatic change in energy use, farming, housing, transport, trade and diplomacy. As well as the long-trailed goal of enshrining in law a climate-neutral Europe by 2050, the plan will propose cutting greenhouse gas emissions by at least 50% by 2030 (compared with 1990 levels). The current target is 40%

Making trade deals only with countries that stick to their climate targets under the Paris agreement

Energy efficiency and renewable energy laws will be tightened up to reflect the more demanding climate targets

To avoid penalising European steelmakers and other industry who are cleaning up their act, non-EU competitors importing energy-intensive goods could face a tax, known as “a carbon border adjustment mechanism”

Using EU funds to develop a zero carbon steel industry by 2030.

Car and truck makers will have to do more to curb emissions, while the EU will seek to boost funding for electric vehicle charge points. Container and cruise ships, currently a gaping hole in efforts to tackle the climate emergency, will be brought into Europe’s emissions trading system, meaning maritime companies will probably have to buy pollution permits

A new strategy to protect European nature, as well as plans to plant and restore forests. Also proposed are plans to improve air and water quality, as well as a review that could lead to tightening of industrial pollution laws

Targets to cut pesticides, chemical fertilisers and increase the area of land devoted to organic farming will be part of an overhaul of the EU’s €59bn a year common agricultural policy

A plan to raise a €100bn “just transition fund” from public and private sources to help EU member states move beyond fossil fuels The European Green Deal is only the start of the journey: laws will have to be drafted and agreed by EU ministers and MEPs; money will have to be raised; plans will have to be implemented. It is a route map, rather than a destination, and time is running out.

Poland, which relies on coal for 80% of its electricity, faces a massive bill to wean itself off fossil fuels. Its government is angling for a slice of the €100bn fund to be earmarked for Polish needs alone – a request deemed unacceptable by other member states. While Hungary does not have the same coal dependency as Poland, it is also taking a maximalist position on EU funds.

But the Czech prime minister, Andrej Babiš, is seen by some diplomats as the biggest wild card. Babiš is under pressure at home, after an EU audit leaked earlier this month revealed he was in conflict of interest over EU funds received by his Agrofert conglomerate. The Czech prime minister was one of the strongest voices against the 2050 target in June; the latest negative report from Brussels on his business interests is seen as making him an unpredictable player in the debate.

Arriving at the summit, Babiš said reaching carbon neutrality by 2050 would cost his country €30-40bn. “We have also the question, how will we force China and Russia and India to decrease emissions?”

He is one of several leaders arguing for nuclear energy to be flagged as part of the EU answer to carbon neutrality – a proposal that has upset anti-nuclear governments in Austria and Luxembourg.

Lithuanian president Gitanas Nausėda, who supports the 2050 target, said it would cost his country two thirds of its GDP until 2050. “Otherwise, it will be very difficult to be competitive [to] spend money on green economy if the third countries don’t do this.”

Germany’s chancellor Angela Merkel said she hoped all countries could agree on the target: “It would be a strong signal that Europe is really the continent that is climate-neutral in 2050.”

French president Emmanuel Macron also voiced hope other countries would back the target and made the case for a tax on foreign imports not meeting EU environmental standards “to avoid penalising our industries”.

The target was also supported by Finland’s leader, attending her first EU summit since being sworn in earlier this week. “The new generation are expecting us to act and we have to fulfil the expectations of the people,” said Sanna Marin, who at 34 is the youngest prime minister in the world.

Boris Johnson is not taking part in the summit because of the ongoing election and will be represented by Charles Michel, who is chairing his first EU leaders’ summit since becoming European council president this month.

Officials have been wrangling over a summit communique to create what one source called “a smart linguistic solution” to get everyone on board. The latest draft summit conclusions seen by the Guardian call for an “enabling framework” to support “a cost-effective, just, as well as socially balanced and fair transition taking into account different national circumstances”.

Climate activists argue that a promise on the 2050 target alone is not enough and want the EU to set a more demanding emissions reduction target for 2030.

“The world is on fire and our governments are letting it burn,” said Greenpeace’s EU director, Jorgo Riss, in a statement. “It’s not enough for them to commit to a climate neutral EU in 2050. The presidents and prime ministers in Brussels today will be long gone by then. What counts even more is the urgent action they take now, while they are in power.”

He said the European green deal was a “starting block” and called for an end to subsidies for oil and gas companies, a move to 100% renewable energy as fast as possible, measures on sustainable transport and energy savings, as well as action to restore the countryside, forests and oceans.