CANBERRA (REUTERS) - The quality of air in Australia's capital of Canberra was the worst of any major city in the world on Monday (Jan 6) as smoke drifted in from fires, prompting shops and offices to tell staff to stay home and the national gallery to close its doors to protect its art.

While not in any imminent danger from fire, Canberra sits between blazes ravaging the east coast and others inland, and its skies have been darkened by smoke this week.

The National Gallery of Australia said it would not open on Monday in part to protect works being shown in its Matisse and Picasso exhibition, including some borrowed from the Musée Picasso in Paris.

"Closing our doors allows us to mitigate any risk to the public, staff and works of art on display," the gallery said.

The exhibition includes Picasso's portraits of his wife, Olga, and lover, Marie-Therese, both on loan from Paris, it said.

Southern-summer fires have devastated more than 8 million hectares of Australia's bushland and killed at least 24 people and countless animals.

Acrid, yellowish smoke has blanketed towns and cities, raising concern about public health.

Canberra health authorities warned the city's 400,000 residents to avoid outside physical activity.

Many businesses and government departments, including the Department of Home Affairs, ordered non-essential staff to stay home, media reported.

Air quality readings show the smoke was at hazardous levels early on Monday - and the worst among all major cities - before wind dispersed some of it during the day.

Nasa images have shown smoke billowing from Australia's east coast drifting over New Zealand and even reaching South America in less than a week.