LONDON - Approachable officers keeping a low profile and self-policing by fans are the keys to a trouble-free World Cup, a study based on Euro 2004 said on Thursday.

The report focused on the behavior of England fans at the 2004 European championship in Portugal and drew on the experience of British and Portuguese authorities, the local police, fans and from direct observations.

The smooth running of the 2004 competition followed violent scenes involving England fans at two tournaments in Europe -- the 1998 World Cup in France and Euro 2000 in Belgium and the Netherlands where more than 1,000 fans were arrested.

No England fans were arrested in the cities where England played in Portugal although some 50 were held in Albufeira, a tourist town a good distance from where the team were playing.

The study, led by Clifford Stott of the University of Liverpool, said the policing success was due to low profile officers mingling with fans, small groups of plain clothes officers looking for any potential trouble and riot police keeping out of sight.

This resulted in England fans seeing their relationship with the police as positive and, as a consequence, they were willing to turn against anyone who was looking to cause trouble.

"Because of this psychology, England fans began to self-police and it was this social psychological process that prevented disturbances," Stott said.

The force in charge of the country's main cities, the Policia de Seguranca Publica (PSP), employed low profile policing after listening to the advice of Stott and the researchers, he said.

In contrast, the country's second police force, the Guarda Nacional Republicana (GNR) in charge of Portugal's smaller towns, employed a different strategy.

In Albufeira, they used riot police to quell minor incidents which appeared to target England fans in general and not just those responsible for the problems, the report said.

"This had the effect of creating and supporting antagonisms between England fans which in turn supported the influence of hooligans and undermined those England fans who wished to prevent problems," the report said.

"Policing in Albufeira was relatively 'high profile' at critical times and resonated with England fans' previous experiences of 'indiscriminately violent' police action."

German police have said they are keen to repeat the positive experiences in Portugal during the June 9 to July 9 World Cup.

More than three million spectators will attend the 64 World Cup matches, spread among 12 venues. Some 1.5 million foreign fans are expected to attend, many of them without tickets.

The report was conducted by the University of Liverpool and the Police Academy of the Netherlands. It was published in the European Journal of Social Psychology on Thursday.