The Associated Press

AP Photo/Lefteris Pitarakis Firefighters work on the rooftop of the hospital, Wednesday.

LONDON — A serious fire broke out at a leading cancer hospital in London on Wednesday, forcing patients and staff out into the cold while scores of firefighters fought the flames and clouds of smoke billowed across the capital's skyline.

No casualties were reported in the blaze at Royal Marsden Hospital in London's Chelsea neighborhood. The London Fire Brigade said it was a "very serious" blaze that broke out on the top floor.

"The top floor and the roof is alight and it's still ongoing. We're doing our best to bring it under control," said a fire department official, speaking on condition of anonymity in line with official policy.

The London Ambulance Service said its crews were remaining on standby but that there were no reports of any casualties so far. The hospital said 79 patients had been moved to alternative locations.

Five operating theaters and at least two wards are very badly affected, said Cally Palmer, the hospital's chief executive.

Television broadcasts showed serious damage to the 150-year-old building and patients being evacuated into nearby streets.

Several patients were laid in an ambulance area on mattresses, being attended to by nurses who were covering them with extra blankets. Other patients were seen being led to safety wrapped in blankets, and in wheelchairs. There were no immediate figures for the number of patients evacuated.

Seventy-five firefighters fought the blaze with powerful hoses. Smoke from the building could be seen across the London skyline.

The Royal Marsden was the first hospital in the world dedicated to the study and treatment of cancer, welcoming patients from across Britain and overseas.

"It's unbelievable what would happen if it burned down. It would be a huge, huge step back for all the patients being treated there, and a huge, huge step back for cancer," said Professor Ray Powles, former head of hemato-oncology at the hospital.