SAN’A, Yemen — Yemen’s chaos deepened Monday when people looting a munitions factory in an area torn from government control by Islamist militants set off an accidental explosion that killed at least 78.

The militant seizure of the factory and nearby towns amplified Western fears that the fragile Yemeni state could deteriorate quickly because of President Ali Abdullah Saleh’s standoff with an opposition coalition of youth groups, military defectors, clerics and tribal leaders calling for his ouster.

Saleh has cooperated closely with the United States in the battle against Yemen’s branch of al-Qaida, which has used areas of Yemen long out of state control to launch attacks.

State control diminished sharply this month as massive demonstrations spiraled in major cities and the government pulled police from many towns, and anti-government protesters set up militias for self-defense.

The protesters blame Saleh for mismanagement, repression and the fatal shootings of protesters, and say they will not relent until he goes.