At least 10 people were killed after a magnitude-5.2 earthquake toppled several buildings in southern Spain near the town of Lorca, officials say.
The quake struck at a depth of just 1km (0.6 miles), some 120km south-west of Alicante, at 1847 (1647 GMT), the US Geological Survey reported.
Lines of cars lay crushed under tonnes of rubble and a hospital was evacuated as a precaution.
The quake followed a 4.4-magnitude tremor about two hours earlier.
It is not clear how many people were injured, although Spanish media say there are dozens.
Spanish TV captured dramatic images of a church bell tower crashing to the ground, landing just metres from a cameraman.
Shocked residents and workers rushed out of buildings and gathered in squares, parks and open spaces. Old buildings were badly damaged.
As night fell many people were still too afraid to return to their homes.
"The whole of the centre of Lorca has been seriously damaged," a delegate from the regional government of Murcia told national radio.
"There are thousands of very disorientated people."
A doctor told the online edition of El Pais that she and her colleagues went into the streets and treated people with serious injuries, many of them "unconscious".
"The ambulances could not reach them. They took more than 40 minutes," the doctor said.
Angel Dominguez, a translator based in Lorca, tweeted: "A friend of ours was in the main avenue of Lorca - she saw debris falling down on pedestrians. The poor girl was shocked."
The earthquakes were felt over a wide area.
"Unfortunately, we can confirm... deaths due to cave-ins and falling debris," Lorca Mayor Francisco Jodar told radio station Ser.
"We are trying to find out if there are people inside the collapsed houses," he added.
A number of aftershocks have been felt in the region after Wednesday's quake, and authorities fear the death toll could rise.
Spain's Prime Minister Jose Luis Rodriguez Zapatero has deployed emergency military units to the scene, the Spanish news agency Efe reported.
Mr Zapatero was in a meeting with Spanish King Juan Carlos when he was informed of the quake, the premier's office said in a statement.
The BBC's Sarah Rainsford in Madrid says the quake is the most serious to hit Spain in about 50 years.
Spain has hundreds of earthquakes every year but most of them are too small to be noticed.
Murcia is the country's most seismically active area and suffered tremors in 2005 and 1999.
Murcia is close to the large faultline beneath the Mediterranean Sea where the European and African continents meet.