BERLIN  Europe’s smoldering financial crisis flared up on Wednesday, with riots over austerity spending in Greece, new signs of troubles in Spain and little indication that European leaders were moving any closer to agreement on a systemic approach to long-term stability.

The day’s events emphasized the complex social, political and economic challenges facing government leaders at a European Union summit meeting on Thursday and Friday in Brussels. The meeting is expected to focus on the financial crisis, but there was no sign of the emergence of the sort of comprehensive plan that financial experts say is needed to beat back the unfolding turmoil.

In remarks to the German Parliament on Wednesday, Germany’s chancellor, Angela Merkel, tried to reassure the markets and answer some of her own critics by allaying fears about the future of the 16-nation monetary union.

“No one in Europe will be left alone, no one in Europe will be abandoned,” Mrs. Merkel said, offering an olive branch to her European partners, some of whom have questioned her commitment to the union. “Europe succeeds when it acts together and, I would add, Europe succeeds only when it acts together.”