Paraguayan footballer Sixto Rojas collapsed during a training session on Wednesday and died almost immediately afterwards, a hospital spokesman said.

"The cause of death is probably a brain embolism," said Roberto Almiron, a doctor at the San Jorge hospital to which Rojas was taken.

The 24-year-old midfielder, who had also played in Chile, was training in a public park with newly-promoted first division club Sportivo Trinidense when he collapsed, his close friend Roque Avalos told a local radio station.

"We're stunned," Avalos said. "Sixto had dedicated himself to a career in football and his death is a tragedy.

"We want to draw attention to the sporting and health authorities over this incident."

Avalos said Rojas was taking part in a running session when he became separated from the rest of the group. He had collapsed when he was found and was taken to hospital in a police car, Avalos said.

Almiron said Rojas arrived at the hospital without any vital signs, although an attempt was made to revive him.

Avalos, known for his pinpoint crosses and free kicks, made his first division debut in 2004 with Olimpia and was transferred to Chilean first division side La Serena in 2005.

He returned home to play for 12 de Octubre last year and joined Trinidense for the 2007 season.

The tragedy sparked controversy about the situation at Paraguay's professional clubs, many of whom work on shoestring budgets, and their training methods in the summer where temperatures routinely climb over 35 Celsius.

"We have to understand that you can't train at certain times in that type of heat," said Carlos Kiese, a former Olimpia coach.

"The boys train in the midday sun, there's no control and sometimes the clubs are even stingy with the water."