Fears that the EU's very foundations and cohesion are threatened by a Polish rejection of Brussels' legal supremacy overshadowed a summit by the bloc's leaders on Thursday.

The gathering -- originally called to examine ways for Europe to cope with a global energy crunch -- exposed deep east-west divisions and dug-in positions that presaged an escalating confrontation in the months ahead.

The seriousness of the row was voiced by several leaders as they arrived, with most emphasising "dialogue" to defuse the situation before it blew up into a political and legal crisis.

"It's very clear that a red line has been crossed," said Belgian Prime Minister Alexander De Croo.

It was "a shame", he said, that so much summit time had to be taken up on the matter, but it was necessary "because this discussion really goes to the heart of Europe".

Dutch Prime Minister Mark Rutte and Finnish counterpart Sanna Marin both said they had to get "tough" with Warsaw, while Austria's Chancellor Alexander Schallenberg warned Poland cannot "cherry-pick" EU laws.

Several leaders said Brussels should not release 36 billion euros ($42 billion) in pandemic-recovery money that Poland badly wants while the issue was unresolved. 

A few said all EU budget money for Warsaw should be subject to an untested "conditionality" mechanism tying disbursement to member states upholding rule of law.

- Defiant Polish PM -

As he arrived, Polish Prime Minister  Mateusz Morawiecki showed no sign of backing down.

While he said he was "ready for dialogue" he warned: "We won't act under the pressure of blackmail."

He stood by a controversial October 7 ruling by his country's Consitutional Court that declared EU law could only apply in just a few, specific areas, and Polish law prevailed in all other national matters.

EU leaders, officials and diplomats see that verdict as a gambit to justify moves by Morawiecki's populist government to get rid of independent judges and replace them with ones controlled by his ruling Law and Justice (PiS) party. They accuse Warsaw of rolling back EU democratic norms.

Morawiecki reiterated his belief that the EU was overreaching in its influence over member states, trampling their sovereignty in a way that would lead to "anarchy" and "chaos" if unchecked.

He found support from Hungarian Prime Minister Viktor Orban, who called the pressure on Poland a "witch-hunt". "The Poles are right," he said, adding "we are going to support the Poles -- there is a devious abuse of authority happening".

In a bid to head off the dispute careening out of control, key leaders hastily organised one-on-one meetings with Morawiecki in the two hours before the summit started.

French President Emmanuel -- who spoke with the Polish leader as soon as both touched down at Brussels' airport -- urged Morawiecki to enter dialogue "to find a solution in line with our principles and common rules," according to an Elysee official.

German Chancellor Angela Merkel and Spanish Prime Minister Pedro Sanchez followed up with their own separate meetings.

- 'Fundamental values' -

European Commission chief Ursula von der Leyen, who publicly clashed with Morawiecki on the issue earlier this week on the same podium in the European Parliament, said the Polish ruling added "a new dimension" to the dispute over rule of law.

"We all have to take a responsibility when it comes to protecting our fundamental values," said von der Leyen, who heads the executive tasked with protecting the EU's treaties.

The summit's convener, European Council President Charles Michel, emphasised the need to find a solution.

"We are firm on the principles of rule of law. We feel that we have tools -- legal tools, institutional tools -- that we should use. But we think also that we must be committed to the dialogue," he said.

Merkel -- attending what could be her last EU summit before handing Germany's reins over to a new government following September elections she did not contest -- said she did not want to see the row end up before the European Court of Justice.

"A cascade of legal disputes before the European Court of Justice is not a solution to the problem of how the rule of law can be applied," she said. 

The European Parliament, however, is already gearing up for legal action to force the Commission to use the conditionality mechanism on Poland.

But the Commission is taking its time. It stresses that a high burden of proof is needed if, as likely, the matter goes to court.

Already the European Court of Justice is using an expedited procedure to decide the legality of the mechanism itself.