The fire broke out just after 9 p.m. local time while the building known as the Manezh was unoccupied.
Two firefighters were killed and one injured from inhaling poisonous fumes while combatting the blaze, said Viktor Beltsov, a spokesman for the Emergency Situations Ministry. The two firefighters had been in the building's attic when it collapsed, the ITAR-Tass news agency reported, citing city fire department spokesman Yevgeny Bobylev.
It wasn't immediately clear what caused the fire, which broke out in the wooden rafters and beams of the roof, Moscow Mayor Yuri Luzhkov said. He said the ventilation system may have been at fault.
ITAR-Tass and the Interfax news agency cited unidentified sources in the Emergency Situations Ministry as saying the cause was probably a short circuit, and Interfax reported that workers had been doing welding on the roof earlier Sunday.
Moscow prosecutors opened a criminal case on the basis of a violation of fire safety rules, Interfax said.
Within an hour after the fire broke out, about half of the 48,000-square-foot structure was in flames and about 3,200 square feet of the roof had collapsed, Luzhkov said. The roof later caved in completely.
Wind carried sparks across the wide avenue running alongside the Manezh, and the high temperatures caused windows in a building housing the journalism faculty of Moscow State University to shatter, ITAR-Tass reported. Luzhkov said none of the adjacent buildings were in danger.
The low, colonnaded Manezh, an equestrian school for military officers that was opened in 1817, is just outside the Kremlin's western wall. It was used for frequent exhibitions and trade fairs and is considered one of Moscow's most precious historic monuments.
The structure is next to a vast underground shopping mall, the Okhotny Ryad, where a bomb explosion in August 1999 killed one person and injured dozens.
