SANAA, Yemen — An explosion ripped through crowds of looters in a munitions factory Monday, killing at least 78 and injuring scores in the latest sign of weakening government authority amid Yemen’s two-month-old populist uprising.

The accidental blast came after soldiers abandoned the factory, allowing the looters to enter — one of a series of incidents in recent days in which government forces have left their posts. They include areas in the north, where rebels have long challenged the state’s authority, and the south, where al-Qaeda’s Yemen branch and secessionists have confronted government forces for years, according to Yemeni and opposition officials, diplomats and local news reports.

The spreading instability underscores growing concerns by the United States and its allies that this fragile but strategic Middle Eastern nation could collapse if its political tensions are not peacefully resolved. It comes as high-level negotiations between President Ali Abdullah Saleh and his opponents over a handover of power have stalled in recent days. Saleh declared that he would give no more concessions to the opposition.