Sao Paulo was blackened for around an hour on Monday after strong winds and a cold front brought in smoke from forest fires burning in the states of Amazonas and Rondonia, more than 2,700km away.
The hashtag #prayforamazonia has now emerged as a global Twitter trend with more than 150,000 references to the fires.
Sao Paulo resident Gianvitor Dias told the BBC what it was like in the city during the smoke-filled blackout on Monday afternoon.
“It was as if the day had turned into night,” he said. “Everyone here commented, because even on rainy days it doesn’t usually get that dark. It was very impressive.”
Some of the most popular tweets online are criticising President Jair Bolsonaro for not taking action.
Scientists from Brazil’s National Space Research Institute (INPE) say the Amazon has suffered losses at an accelerated rate since Mr Bolsonaro took office in January.
The president responded by saying the data “doesn’t relate to the reality” and accused them of smearing the country’s reputation abroad.
The president’s denial of the data which the Institute says is 95% accurate, is something that is being picked up by social media users.
In a tweet that’s been liked more than 10,000 times Twitter user Bea commented that Jair Bolsonaro does not care about the Amazon rainforest. She goes on to express concern at the lack of media coverage towards the fires. She says “people are not talking about it… our greatest natural heritage is being destroyed! We need to wake up and see the reality!”
Other people are voicing their concern about damage the fires are doing to the environment and to nature in one of the world’s most iconic ecological areas.
In a post that’s been liked 17,000 times, Twitter user William Barros highlighted how the world had celebrated Brazil’s natural environment at the opening ceremony of the Rio Olympic Games, only to see it burning to the ground today.
He said: “Three years ago the world saw Brazil celebrating nature and encouraging environmental preservation at the opening of the largest sporting event on the planet.
“Today we see the Amazon, our greatest wealth, on fire for over 16 days. Setback? #PrayforAmazonia.”
The state of Amazonas declared a national emergency earlier this month because of the forest fires, the fourth most affected area in Brazil this summer.
NASA satellite images show the extent of smoke coverage across swathes of Brazil, Bolivia and Paraguay.
The fires have come at a time when concerns are being raised over the levels of deforestation taking place in the Amazon.