MADRID (Reuters) - At least 17 people were injured, most of them lightly, in a strong explosion that rocked a four-storey building being renovated in central Madrid on Friday, Spanish authorities said, and rescue teams were searching for people potentially trapped inside.

       Four people were taken to hospital, one in serious condition, Madrid Mayor Jose Luis Martinez-Almeida said, adding that the cause was not yet clear. "Some kind of works were being done in the building" in the upmarket Salamanca neighbourhood where the explosion occurred, he said.

       "Firefighters are inside checking the building's structure and to determine whether more people could be inside," he told Telemadrid TV channel.

       Emergencies service spokeswoman Gema Martin said that a school across the road from the building was not damaged and that there was no need to evacuate the children.

       (Reporting by Emma Pinedo, Belen Carreno, writing by Andrei Khalip and Inti Landauro, Editing by William Maclean)