Portuguese newspapers published blow-by-blow photographs of the fracas which is thought to have begun after a tackle by Thomas on the Ukrainian Sergei Kandaurov. The players scuffled until other players pulled them apart. The Benfica vice-president Jose Capristano described the spat as "a normal situation."
"It just shows that the players approach training sessions as if they were official games," he said. He added that the clash was "unpleasant".
Benfica, managed by Graeme Souness, currently lie joint second in the Portuguese First Division, one point behind Porto.
Roger Milla has criticised the state of the game in Cameroon describing the national side's recent performances as "lamentable". In the government- run Cameroon Tribune the former World Cup striker, now an assistant coach in Montpellier, France, said: "There is a need for us to bring back the soccer of the old days." He said that he would not return if approached to manage a Cameroon side. "I would prefer to stay in Montpellier, because I feel better there."
In recent years, Cameroon's Indomitable Lions have failed to find the form which took them to the 1990 World Cup quarter-final. They were eliminated in the first round of the 1994 and 1998 finals. Milla captained the 1990 team, scoring four goals, retired from the national side after the 1994 finals.
Following a series of financial problems, Cameroon football has faced trouble off the pitch. Last November football's world governing body, Fifa, dissolved the country's federation and appointed an interim executive to run the organisation for nine months.
In January, Fifa suspended the federation, excluding club and national sides from international competition. The suspension was lifted four days later when the government withdrew reservations over a previously agreed plan of action for running Cameroon football until a replacement executive is elected.