MOSCOW — A dense smog from raging wildfires shrouded Moscow on Friday, grounding flights at the city's airports, plunging its iconic Red Square into a sea of dirty mist and stinging eyes and throats across the city.

Flocks of tourists had to don face masks just to trod the square's historic cobblestones, photographing 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 the Russian capital.

"It hurts my eyes," student Valeriya Kuleva said on a central Moscow street. "I'm wearing a mask but nothing helps."

Dozens of incoming flights were diverted from the capital's Domodedovo and Vnukovo airports, as smog brought runway visibility down to 200 metres, airport officials told The Associated Press. All incoming flights to Domodedovo were being offered alternative airports at which to land, but the decision to divert was up to individual flight crews, airport spokeswoman Yelena Galanova said.

Moscow's other main airport, on the opposite side of the city from most of the blazes, freed up tarmac space to receive some planes. Other flights decided to divert to St. Petersburg, 640 kilometres to the northwest or to Kazan, 800 kilometres east of Moscow, a Vnukovo Airport spokeswoman Irina Ivanova told The AP.

Visibility in parts of the capital was down to a few dozen metres due to the smog, which carries a strong burning smell and causes coughing. Kremlin buildings and church domes disappeared into the haze, which is forecast to hang in the air 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."

More than 500 separate blazes were burning nationwide Friday, mainly across western Russia, according to the Emergencies Ministry. Dozens of forest and peat bog fires around Moscow have ignited amid the country's most intense heat wave in 130 years of record-keeping.

"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.

He added 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 have evacuated explosives from military facilities and were sending planes, helicopters and even robots to help control blazes around the country's top nuclear research facility in Sarov, 480 kilometres east of Moscow.

A wildfire last week caused huge damage at a Russian naval air base outside Moscow, with Russian media reporting as many as 200 planes may have been destroyed.

The forecast for the week ahead, with temperatures approaching 38 C, shows little change in Moscow and surrounding regions, where the average summer temperature is around 23.