Firemen struggled to contain flames leaping half a kilometre into the sky from a blaze near the top of Moscow's Ostankino television tower that crippled broadcasts across Russia.
Four people, including three firefighters, were reported to be trapped in a lift amid fears that the top of the 540-metre (1,772 ft) tower, the world's second-tallest structure, could collapse.
"Nothing like this has ever happened before," said Mr Mikhail Serikov, the head of a team of firefighters.
A firefighters' spokesman, Mr Nikolai Starychev, said that rescue crews were trying to locate alift with four trapped people, three firefighters and a woman who operated the elevator.
Mr Starychev said power went off when the four were at a height of 271 metres on their way up to deliver supplies to firefighters. They risked the lift instead of taking many flights of stairs.
Thick grey-and-white smoke billowed from the upper parts of the tower, which was the tallest building in the world when it was completed in 1967. Only the CN Tower in Toronto is taller.
Police set up barriers forcing thousands of spectators to stand more than 700 metres away in case parts of the building collapsed.
The tower was once a source of pride for the Soviet Union as a symbol of modernity, and to this day foreigners still pay three times more than Russians to visit it. The Ostankino facilities were the site of fighting in 1993 when rebels tried to topple the government of President Boris Yeltsin.
The fire briefly blacked out national television across the world's largest country, as state broadcasters RTR and ORT and the private NTV went off the air.
Broadcasting resumed to most regions by satellite soon afterwards, but Moscow screens remained blank yesterday evening except for cable television subscribers and those able to receive two small channels on UHF frequencies.
A number of radio stations also went off the air, and other communications such as pager services were also disrupted. The tower also links Russia's police ambulance and fire services.
The fire began in a technical area above the Seventh Heaven restaurant which offers a panoramic view of the city. Diners were told to leave the 334-metre high restaurant through an emergency exit.
A fire safety service spokesman said the fire was burning in the tower's electronic cabling. RTR television said a short circuit was the likely cause. The spokesman said officials would try to get firefighters to the top of the tower from the helicopter to fight the fire from above.
Reporters saw a helicopter try to pipe fire retardant into the tower.
President Vladimir Putin met the Communications Minister, Mr Leonid Reiman, to discuss how to restore television service, the Kremlin said.
The tower is made of reinforced concrete, and the interior has a bank of elevators and a single emergency stairwell. Other parts contain banks of radio and television transmitters.
Russian media speculated whether the tower would be able to resist the fire, but firefighters and experts said they were optimistic.
"There is a chance there will be deformations in the tower, but experts are pretty sure it can resist the fire," Mr Starychev said.