The death of a football fan in a pre-match ruckus that saw rioters marauding with metal bars and knives in Madrid raised tough security questions for Spanish authorities on Monday.
Radical fans on the fringes of four Spanish clubs apparently planned Sunday's brawl using a mobile phone group messaging service, escaping police detection, officials said.
The clash ahead of Atletico Madrid's 2-0 win over Deportivo de La Coruna drew 200 members of hardline groups linked to the two sides plus two other Madrid clubs, Rayo Vallecano and Alcorcon.
When it was over, 43-year-old father of one Francisco "Jimmy" Romero was pulled out of the Manzanares river having had a heart attack while he also was suffering from hypothermia and head injuries and died soon afterwards.
On Monday, 21 people were still in police detention.
"There were no indications that could have allowed us to foresee such incidents," said Spain's junior security minister Francisco Martinez after a meeting on Monday of the country's Antiviolence Commission.
"They did what was necessary to deliberately escape the control of the police."
A video camera captured the moment when a man identified by Spanish media as Romero -- a member of the Deportivo hardcore fan group Riazor Blues -- was heaved over the embankment into the river by his adversaries near Atletico Madrid's Vicente Calderon stadium.
Medics said at least 11 other people were injured, including a police officer.
- Rioters evaded police -
Police sources said rioters from La Coruna travelled in a coach hired in a different city to avoid being identified as Deportivo fans.
The rival groups arranged to meet for their brawl by exchanging messages on WhatsApp, a mobile telephone application that is less easy to monitor than other online social networks.
Police had categorised Sunday's match as low-risk since there had not been any incidents between fans of the two sides in the past six years, officials said.
Of the 21 people still in detention on Monday, four were from Frente Atletico, a group linked to fans of the Spanish league champions, and 14 from Riazor Blues, a police spokesman told AFP.
One was from the Alcorcon group Alkor Hooligans and two from the Bukaneros of Vallecas.
The two latter groups are considered politically extreme-left and are thought to have teamed up with Riazor Blues to fight against the far-right Frente Atletico.
"These groups meet up and form alliances. They do not act according to their sporting allegiances but rather to their ideological affinities," said Miguel Cardenal, Spain's junior sports minister.
Spain's opposition Socialist Party demanded that Interior Minister Jorge Fernandez Diaz face questions in parliament about the level of police surveillance in Madrid on Sunday.
The BNG nationalist party from the Galicia region where La Coruna is located also demanded an explanation, alleging an absence of police ahead of the match.
"What happened was absolutely deplorable," said Spain's Prime Minister Mariano Rajoy on Monday, speaking during a visit to Paris.
"It is very important that everyone -- government, clubs and the media -- get involved" in solving the problem, he added.
"Some clubs have made an effort by themselves to eradicate such behaviour,"  he said, alluding to Real Madrid and Barcelona.
Barcelona in 2003 banned the Boixos Nois radical fan group and last January Real Madrid did the same to members of the extremist collective Ultrasur.
Cardenal said authorities were considering possible sanctions such as partially closing stadia or banning radical groups from them.
In 2003 a Deportivo fan died after being beaten by radical fans of his own team as he tried to protect a boy wearing a rival side's shirt.
A Real Sociedad fan, Aitor Zabaleta, died after being stabbed near the Calderon stadium prior to a match in December, 1998. A right-wing Atletico radical was jailed for his murder.
Source: AFP