The alliance of parties from four distinct political backgrounds, nicknamed after the famous composer's "Four Seasons", follows 653 days of temporary arrangements.
In a thinly-veiled reference to the UK, EU chief Charles Michel said "respect for treaties, a basic principle of international law, comes to be considered optional even by those who, until recently, were its historical guarantors".
"This won't be easy, as there are different interpretations on the table, but I'm totally convinced that we'll find a solution," European Council President Charles Michel told Euronews.
EU leaders will on Thursday "advance altogether on a very important topic, that of our climate ambition", French President Emmanuel Macron said upon arrival in Brussels.
The presidency of the European Council has been officially handed over to Charles Michel. The former Belgian Prime Minister will chair the meetings of EU leaders for the next 5 years.
The Belgian politician takes over from Tusk at the top table. We look at his path to power, and the skills he could bring to the role of EU Council president.
Donald Tusk is replaced as President of the European Council by former Belgian Prime Minister Charles Michel.
The nationalist right and extreme right prevailed in the Flemish north of the country. In Wallonia, the majority voted for the socialist party and the greens.
Belgian prime minister forms a minority government after biggest party in coalition quits over UN migration treaty row.
The anti-tax 'gilets jaunes' movement has spread from France to Belgium, with protesters in Brussels marching against economic inequalities.