Witnesses described a high-intensity explosion
A van loaded with a bomb has exploded near a conference centre and railway line in the east of Spain's capital, Madrid, reportedly causing no injuries.
The bomb exploded outside the Campo de las Naciones at around 0900 (0800 GMT), about 90 minutes after a warning was received by the Spanish Red Cross.
Police were able to cordon off the entire area and clear the trade fair centre before the blast occurred.
The Spanish authorities have blamed the Basque separatist movement, Eta.
Eta has been blamed for the deaths of more than 820 people and for numerous car bombs during its 40-year campaign for an independent Basque nation.
The Spanish Red Cross received a telephone call shortly at 0737 (0637 GMT) on Monday morning, naming the street in eastern Madrid where the bomb had been left inside a parked Peugeot van.
The device exploded an hour and a half later, close to the Campo de las Naciones centre, a railway line, and the headquarters of the construction firm Ferrovial Agroman.
Witnesses have described a high-intensity explosion which broke windows. Emergency services at the scene have reported no injuries.
The attack happened less than three weeks before regional elections in the Basque Country.
At the weekend, the Spanish Supreme Court banned two nationalist parties from fielding candidates in those polls following allegations of links to Eta.
The BBC's Steve Kingstone in Madrid says Spanish police had been expecting a violent response.
Railways have been a chosen target of Eta, which opposes the construction of a high-speed line between Madrid and the Basque Country, our correspondent says.
Last month, the group said it was behind the December killing of a Basque businessman whose company was working on the project.