A football-sized block of ice fell out of the sky on to the roof of an industrial building in Madrid, police say, denting the roof but without causing injuries.

"We don't know where it came from," a police spokeswoman said of the 20 kilogram lump.

"We've taken samples from the block which are in the process of being analysed."

A series of ice blocks fell around Spain in 2000.

Scientists have said the most likely explanation is a sudden freeze in the upper part of the Earth's atmosphere forming ice blocks that then fall into a restricted area.