GAO, Mali — Black-robed Islamist extremists armed with automatic rifles invaded Gao in wooden boats Sunday to launch a surprise attack on the most populous city in northern Mali, two weeks after French and Malian troops ousted the jihadists.

Gunfire echoed for hours across the city of mud-walled buildings. The combat started at about 2 p.m. in downtown Gao and the fighting was continuing as night fell. Later the sound of gunfire was replaced by the clattering of French military helicopters overhead.

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The attack in Gao shows the Islamic fighters, many of them well-armed and with combat experience, are determined and daring, and it foreshadows a protracted campaign by France and other nations to restore government control in this vast Saharan nation in northwest Africa.

The Islamist radicals fought against the Malian Army throughout the afternoon and were seen roaming the streets and on rooftops in the center of Gao, which had a population of 90,000 before the conflict caused thousands to flee.

The fighting seemed to center at police headquarters, where Malian soldiers with rocket-propelled grenades traded fire with combatants.