The police advised residents in the area of Triq l-Ibraġ, Ibraġ to keep their windows closed as a grass fire in Wied Għomor enveloped the area in smoke on Monday afternoon.
Motorists were also urged to avoid the area.
The fire broke out at about 2pm and several streets linking Swieqi with Wied Għomor were also closed to enable firefighters to battle the blaze, which spread quickly, fanned by a strong wind.
Neighbours reported ash falling on their yards and roofs.
The Civil Protection Department deployed several fire engines. The blaze was brought under control by the evening, but many areas of the valley had been blackened.
"It's terrible, the last green lung in the area destroyed, an eco-disaster, as there was a lot of wildlife thriving there," a Swieqi resident said in a Facebook message to Times of Malta.
The police said no one was injured.