Flocks of tourists had to don face masks just to tread the square's historic cobblestones, straining to photograph the Kremlin's barely visible spires and the hazy domes of St. Basil's Cathedrals.
Airborne pollutants such as carbon monoxide were four times higher than average readings the worst seen to date in Moscow, city health officials reported.
"It hurts my eyes," student Valeriya Kuleva said on a central Moscow street. "I'm wearing a mask but nothing helps."
Dozens of flights were grounded and others were diverted away from the capital's Domodedovo and Vnukovo airports, as smog brought runway visibility down to 220 yards, airport officials told The Associated Press. All incoming flights to Domodedovo were being offered alternative airports at which to land, a decision left up to individual flight crews, airport spokeswoman Yelena Galanova said.
Other flights decided to divert to St. Petersburg, 400 miles to the northwest or to Kazan, 500 miles east of Moscow, a Vnukovo Airport spokeswoman told the AP.
Sheremetyevo, Moscow's other main airport, on the opposite side of the city from most of the blazes, also faced some delays but freed up tarmac space to receive some planes.
By late afternoon, the situation had eased somewhat, with all airports starting to accept some flights.
Visibility in the capital was down to a few dozen yards due to the smog, which carried a strong burning smell and causes coughing. The haze is forecast to hang around for days due to the lack of wind.
"It's just impossible to work," said Moscow resident Mikhail Borodin, in his late 20s, as he removed a face mask to puff on a cigarette. "I don't know what the government is doing, they should just cancel office hours."
Russian health officials have urged those who have to go outdoors to don face masks and told people staying inside to hang wet towels to attract dust and cool the airflow. The Health Ministry said hundreds have needed medical attention due to the smog.
Ken Donaldson, professor of respiratory toxicology at the University of Edinburgh, said people with asthma, bronchitis, lung disease or heart problems were the most vulnerable to the smog.
"For people with underlying health problems, the particles in the smog could be the straw that breaks the camel's back," he said, causing them to have a serious lung problem or a heart attack.
He said concentrations of carbon monoxide, even at four times higher than normal, was not alarming unless people became trapped in an enclosed space. The more dangerous gases are ozone or sulfur dioxide, he said, but those are not usually produced by burning.
More than 500 separate blazes were burning nationwide Friday, mainly across western Russia, amid the country's most intense heat wave in 130 years.
"All high-temperature records have been beaten, never has this country seen anything like this, and we simply have no experience of working in such conditions," Moscow emergency official Yuri Besedin said Friday, adding that 31 forest fires and 15 peat-bog fires were burning in the Moscow region alone.
At least 52 people have died and 2,000 homes have been destroyed in the blazes. Russian officials have admitted that the 10,000 firefighters battling the blazes aren't enough an assessment echoed by many villagers, who said the fires swept through their hamlets in minutes.
To minimize further damage, Russian workers evacuated explosives from military facilities and were sending planes, helicopters and even robots in to help control blazes around the country's top nuclear research facility in Sarov, 300 miles east of Moscow.
A wildfire last week caused huge damage at a Russian naval air base outside Moscow.
Moscow faces temperatures approaching 100 degrees Fahrenheit for the next week, according to the forecast, in contrast to its average summer temperature of around 75 F.