SANTO DOMINGO, Dominican Republic  A fierce earthquake struck Haiti late Tuesday afternoon, causing a crowded hospital to collapse, leveling countless shantytown dwellings and bringing even more suffering to a nation that was already the hemisphere’s poorest and most disaster-prone.

The earthquake, the worst in the region in more than 200 years, left the country in a shambles. As night fell in Port-au-Prince, Haiti’s capital, fires burned near the shoreline downtown, but otherwise the city fell into darkness. The electricity was out, telephones were not working and relief workers struggled to make their way through streets blocked by rubble.

In the chaos, it was not possible for officials to determine how many people had been killed and injured, but they warned that the casualties could be substantial.

The physical toll was easier to assess. The headquarters of the United Nations mission was seriously damaged, the United Nations said in a statement, and many employees were missing. Part of the national palace had collapsed, The Associated Press reported.