European Union leaders wrestled with German demands for strict austerity and a French-led push for growth-friendly spending at a summit, which was targeted by thousands of protesters angry over rampant unemployment.

With the EU's core eurozone economy languishing in recession, organisers said 15,000 people took part in the rally in a park next to EU headquarters in Brussels, with dozens arrested after breaking into an adjacent building.

The two-day meeting opened with French President Francois Hollande calling on fellow leaders to lift an arms embargo on Syria to help insurgents fighting the regime of President Bashar al-Assad.

Hollande held head-to-head talks with British Prime Minister David Cameron after saying on arrival at the venue: "We are ready to support the rebellion."

The summit will also feature a gathering of eurozone-only leaders overnight, joined by eurozone head Jeroen Dijsselbloem whose Eurogroup of finance ministers also meet later on Friday, tackling a multi-billion-euro bailout for Cyprus – the fifth for an EU state since Greece first needed rescuing three years ago.