The three men and one woman were reported missing by the construction company in charge, Madrid's Deputy Mayor Inma Sanz told reporters.
"The upper floors collapsed and fell downwards, therefore we're talking about a very significant amount of rubble that will take a long time to clear - not just hours, but probably several days," Sanz said.
Police and firefighters were using drones and sniffer dogs to search for the missing inside the building located near the Spanish capital's opera house and the royal palace.
Police are also at the scene.
In video posted on X by emergency services, the building's facade was covered by a huge green tarp typically used by construction crews when renovating older buildings.
The facade of the building was intact, and the rubble had not fallen outside on the street.
Construction worker Mikhail was pumping concrete into the building's lower floors and was outside when the collapse occurred.
He said he saw a large cloud of dust and immediately sprinted away.
"I was the first to run, I didn't care about anything else. I'll save my life first and, if I can, save others later," he told reporters.
with Reuters
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