The 24-year-old, who became the second international footballer in the last seven months to die during a game, had come on as a substitute after 60 minutes of the league match against Vitoria Guimaraes and collapsed in the final minutes.

He was stretchered off the field in Guimaraes and taken to hospital but died shortly afterwards.

However after a four-and-a-half hour autopsy the causes of his death were still not known.

"Tissue was taken from him for additional examinations," read a statement from the prosecutor general's department.

"These will involve the anatomy and for drugs."

Feher had just been booked in injury-time for time-wasting after Benfica had taken a late 1-0 lead.

Football fans watched on television and at the stadium in Guimaraes, located some 320 kilometres (180 miles) north of Lisbon, as doctors tried to save an apparently unconscious Feher as he lay on the muddy field.

His Benfica teammates, including coach Jose Antonio Camacho, wept or prayed as they paced the pitch in the driving rain before an ambulance arrived nine minutes after Feher collapsed to take the player to hospital where he was declared dead at 11:10 p.m.

(2310 GMT).

Benfica and Guimaraes said in a joint statement Feher was treated for a heart attack for about 90 minutes before he died.

Once the autopsy was completed, Feher's body was taken to Lisbon and will lie in a chapel at Benfica's new 65,000-seat Stadium of Light where fans have been laying candles and flowers since his death.

Feher's parents, brother and fiance arrived in Portugal on Monday and will accompany his body back to Hungary where the player's funeral will take place on Wednesday.

The player was reportedly set to get married in June. Benfica said in a statement it would rent a plane to take the entire squad to Hungary for the funeral.

"He was very well-liked, very polite," Feher's manager Jose Veiga told Portuguese radio TSF.

"He had absolutely no malice."

The Portuguese Football Federation said there would be a minute's silence at the start of next weekend's league games in honour of the player, who had lived in Portugal since he was 18.

His sudden death dominated newspaper headlines in Portugal on Monday, with all three sports dailies featuring front pages draped in black.

"Death on live television," the daily Diario de Noticias said above a photo of Feher as he fell, while sports daily A Bola featured a photo of the player smiling just seconds before he collapsed above the headline: "The last smile of Miklos Feher." Asked by reporters about the amount of time it took for the ambulance to arrive to take the player off the pitch, Portuguese secretary of state for sport Herminio Loureiro said everything was done to save his life.

"The information I have at the moment indicates everything was done, and I mean everything, at the hospital and stadium to avoid this tragic end," he said.