The acrid smoke from the fires in the countryside around the city is seeping into apartments, offices and even the underground railway system, forcing people to flee the city in droves.
Some 557 wildfires were still burning across 174,000 hectares of land, marking only a slight decline from the weekend, authorities said.
Media reports said well-off Muscovites were spending nights at air-conditioned hotels and accused authorities of covering up the true scale of the environmental disaster.
"Morgues and crematoria are overcrowded," the Kommersant daily quoted an unidentified head of an enterprise in the funerals industry as saying.
A doctor with a Moscow ambulance crew told the Novaya Gazeta daily on condition of anonymity that the number of ambulance calls and deaths had gone up in recent days.
"We have been strictly forbidden to hospitalise people barring the most extreme cases," he said, complaining of hazardous working conditions.
"There are no air conditioners in vehicles and those that are simply do not work. Temperatures inside reach 50 degrees...Sometimes our doctors faint."
A surgeon at a major hospital described a similar picture, saying the smog and heat were taking its toll on both patients and medical staff.
State air pollution monitoring service Mosekomonitoring said carbon monoxide levels in the Moscow air were 2.2 times higher than acceptable levels early on Monday.
Weather forecasters say shifting winds are expected to help clear the air in the middle of the week, while the heatwave would continue for the next few days and subside by early next week.
The heatwave created a national catastrophe which has affected all areas of life, with 10 million hectares of agricultural land destroyed and the government ordering a ban on grain exports.