Rwanda-backed rebels claimed they captured eastern Congo’s largest city, Goma, early Monday, as the United Nations described a mass panic among its 2 million people and Congo’s government said the rebel advance was a “declaration of war.

A rebel alliance spearheaded by the ethnic Tutsi-led M23 militia said it had seized the lakeside city of more than 2 million people.

Thousands fled the city of Goma on Monday as fighting raged between Congolese forces and rebels backed by neighboring Rwanda, who claimed to have captured eastern Congo’s largest regional hub. Pockets of chaos and gunfire had some people hunkering down as the rebels marched into the city with a population of about 2 million.