LONDON, Jan 2 (Reuters) - A fire tore through the top floor of a leading cancer hospital in London on Wednesday, forcing the evacuation of all staff and patients including some already in the operating theatre.

The entire top floor of the Royal Marsden Hospital was gutted, and patients were taken in near-freezing weather to neighbouring hospitals and churches.

Hundreds of staff, some still dressed in surgical scrubs, gathered outside as streets surrounding the hospital were shut.

A London Fire Brigade spokesman described the blaze, which started just after 1:15 p.m. (1316 GMT) in the Fulham Road hospital in southwest London, as "a very big and serious fire".

He said there were 25 fire engines and 125 firefighters tackling the blaze.

"All the patients and staff have been evacuated from the hospital," he added. "It is one of the larger fires we have had to deal with."

There were no reports of casualties or early indications of the cause of the blaze.

Patients were taken to the nearby Chelsea and Westminster hospital and the Royal Brompton hospital. A spokeswoman for the Royal Marsden said all patients were safe. Hundreds of patients and staff were evacuated from the hospital, but there has been no confirmation of exact numbers.

A London ambulance spokesman said 15 ambulances and a Hazardous Area Response Team were at the scene.

A doctor at the hospital, Toni Burke, told Sky News that at least two patients had been in theatre as the blaze took hold.

Patient numbers overall were lower than normal because many had gone home for Christmas.

Television pictures showed thick smoke and flames billowing from the top floor with part of the roof reduced to embers.

Local worker Alicja Kass, 31, told Reuters that smoke had blanketed the area.

"There is loads of smoke -- it is very dark in the streets," the sales assistant said.

According to its Web site, the hospital was founded in 1851 and was the first in the world dedicated to cancer treatment and research into the causes of cancer. (Reporting by Andrew Hough and Jeremy Lovell; Editing by Stephen Addison and Caroline Drees)

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