A Moroccan suspected of involvement in the Madrid train bombings has been arrested in Belgrade, Spain says.
The interior ministry named the suspect as Abdelmajid Bouchar, 22, and said it wanted him extradited to Spain.
He was picked up by Serbian police when he presented false Iraqi identity papers during a routine check, it said.
Mr Bouchar is alleged to have fled an apartment in Leganes, near Madrid, where the suspected train bombers hid out after the attacks in March 2004.
The attacks killed 191 rush-hour commuters and injured nearly 2,000 more.
Mr Bouchar is reported to have been hiding out with seven other suspects in Leganes on 3 April, 2004. The Associated Press news agency quotes investigators who say Mr Bouchar went down to the street to empty some rubbish and spotted the police.
He is said to have shouted out in Arabic to warn his colleagues, before escaping.
Appeal to Interpol
The seven suspects in the apartment blew themselves up when police surrounded the building. A police officer also died.
Mr Bouchar went on the run and was the subject of an international arrest warrant.
Seven suspects blew themselves up in the Leganes flat
Serbian police grew suspicious when the suspect refused to cooperate with their inquires and he was identified through Interpol.
Spanish Interior Minister Jose Antonio Alonso said the arrest showed, once more, the need to strengthen international collaboration to improve the efficiency of the fight against terrorism.
More than 70 people have been arrested in the course of the investigation into the Madrid attacks, which have been blamed on Islamist radicals.