The city of Kazan, on the Volga river east of Moscow, was also blanketed in smog on Monday, an eyewitness told the BBC.
Marek Zaremba-Pike said Kazan's air "smells of burnt wood and tastes of dust".
"Usually we can see the Kazan Kremlin very clearly, but visibility is poor. You can't see it at all, just the river bank."
More famous for its bitterly cold winters, the giant country's European part normally enjoys short, warm summers.
However July was the hottest month on record. In Moscow, which sees an average high of 23C in the summer months, recorded 37.8C last Thursday.