A brainchild of French President Emmanuel Macron, the EPC brings together the 27 member states of the European Union and 17 other countries, including several waiting to join the bloc and the only one ever to leave it, the United Kingdom.
An EU diplomat said the gathering of so many leaders in Prague would send "a very strong signal" to Russian President Vladimir Putin of opposition to the war unleashed in Ukraine after his February invasion.
Diplomats say no such Europe-wide grouping exists, and this is a good moment to forge one as the whole continent grapples with consequences of the war, including the threat to peace and security and an economically devastating energy crisis.
But, on the eve of the gathering, EU foreign policy chief Josep Borrell said with so many leaders meeting for just half a day it could only be "an initial exchange," and there were still unresolved questions about the forum's ultimate goal.
In a blog, Borrell said clarity was still needed on the EPC's core rationale, the forum's final membership, its relationship with the EU, how it should take decisions and even whether it should have a budget of its own.
Some have already written off the EPC as just another talking shop, one that will be difficult to manage not just because of its size but also because of its diversity and the traditional rivalries between many of its members, from Armenia and Azerbaijan to Greece and Turkey.
Still, Mujtaba Rahman, Eurasia Group's managing director for Europe, said it could present an opportunity for the EU to advance its strategic goals on energy supply, migration, security and defence.
He said British Prime Minister Liz Truss's decision to attend the summit could also facilitate a reset of ties between Brussels and London that have been frayed by post-Brexit wrangling over Northern Ireland.
The meeting in Prague Castle, a sprawling complex founded more than a 1,000 years ago that includes a cathedral and cobblestone streets, will be followed on Friday by a meeting of EU leaders.
This summit will be dominated by how to cap gas prices to contain soaring energy costs.
Diplomats said the EU summit would probably also include tensions over Germany's announcement of a 200 billion euro ($197.50 billion) support package for businesses and households that other member states could never afford.