Residents complain of stinging noses and throats.
The quality of the air is so bad that even the normally chirpy tropical birds are silent, they say.
"My nose stings all the time and so do my eyes, even when they are closed. We can't breathe properly," said university student Ronny Gonsalves. "I cannot see the horizon."
Experts say the smog is coming from fires around the city and inside its metropolitan area, often deliberately set to clear trees for agriculture and urban development without permits.
Very dry conditions caused by a severe drought that the region is suffering, with record high temperatures, has helped ignite and spread the fires, according to Carlos Durigan, Brazil's country director for the Wildlife Conservation Society.
"It has never been this bad in decades past," the geographer who has lived for 30 years in the Amazon said.
The region is under pressure from the El Nino weather phenomenon and rainfall in the northern Amazon is well below the historical average. The streams and tributaries that run into the Amazon have fallen to near record levels.
Brazil's government has sent teams of firefighters to contain the forest fires. But residents fear the smoke-filled air will persist until it rains, and that is not expected until the end of November.
Wildfires in the Amazon rainforest have been more common in past years to the southwest of Manaus in the states of Rondonia and Acre.
Durigan said many roads built around Manaus have led to faster deforestation within and outside the metropolitan area. Land grabbers often set fires to illegally clear land, he said.
"Today was the worst day. We can no longer see the river from my apartment," said Maria Lucia de Freitas, who said the smog first appeared over Manaus a month ago.
(Reporting by Bruno Kelly, Writing by Anthony Boadle, Editing by Angus MacSwan)
By Bruno Kelly
