LISBON, Jan 26 (Reuters) - Portugal's soccer chief declared on Monday a day of mourning after the on-field death of Benfica's Hungarian international Miklos Feher, the victim of an apparent heart attack.

The 24-year-old striker collapsed on Sunday during a televised league game, the second international to die on the field of play in seven months. A heart condition killed Cameroon's Marc Vivien Foe at the Confederations Cup in France last June.

"This is a bad day for us," Gilberto Madail, chairman of the Portuguese Soccer Federation, told TSF radio. "Last night was a truly black night."

Under a light rain, hundreds of mourners of all ages gathered at Benfica's Stadium of Light in Lisbon. They formed long lines to sign a book of condolences and lay flowers at the opened main gate.

Feher collapsed on the rain-sodden pitch in injury time of the match against Vitoria Guimaraes with Benfica leading 1-0. Benfica had taken the lead in injury time through Fernando Aguiar.

Referee Olegario Benquerenca blew the final whistle after consulting both captains as there was still a minute or so of injury time to be played. The result is expected to stand.

"Death live," read the front-page headline in Monday's edition of Diario de Noticias newspaper, while Publico newspaper splashed, "Death in Guimaraes".

IN MOURNING

Television showed repeated footage of Feher smiling at the referee after receiving a yellow card. He leaned over with his hands on his thighs, then slumped backwards on to the pitch.

Feher was taken by ambulance to the Lady of the Olive Tree Hospital. Medical personnel gave up efforts to revive him about one hour and 40 minutes after his collapse.

A hospital spokesman said Feher had died of cardio-respiratory arrest.

The attorney general's office said in a statement the autopsy was inconclusive and laboratory tests would be carried out on samples taken from the body, TSF radio said.

The four-and-a-half hour autopsy found no signs of damage and Feher's body was released after the examination.

Benfica players and executives will accompany the body back to Hungary for the funeral on Wednesday, Benfica said in a statement. Feher's parents have also arrived in Portugal.

Benfica and Guimaraes said in a joint statement that Feher had received appropriate medical treatment when he collapsed.

Prime Minister Jose Manuel Durao Barroso and President Jorge Sampaio sent letters of condolence to Benfica or to Feher's family. Benfica is one of Portugal's traditional powerhouse clubs and claims to have millions of fans in the country's emigrant communities.

World soccer's governing body FIFA has drafted letters of condolences to Benfica and the Hungarian Football Association. A spokesman said: "We are deeply saddened by this new tragedy that has struck the game.

"We are obviously as keen as everybody else is to learn the medical causes."

NOTHING UNUSUAL

Hungary FA officials, including national coach Lothar Matthaeus, were on a flight to Malta to discuss dates for World Cup qualifiers and unreachable for immediate comment.

But Dezso Lejko, the Hungary team doctor, said he had never found anything unusual in Feher's health.

"I knew him for four years, since he arrived in the Under-21 team," he told local radio. "I never found anything wrong with his physical state, he had no heart problems at all."

Feher came to Portugal to play for Porto in the 1998/99 season from Hungary's Gyor Eto and moved to Benfica in the 2002/03 season after loan spells at Salgueiros and Braga.

The last top Portuguese player to die during a match was Fernando Pascoal das Neves of Porto in December 1973. He collapsed in the 13th minute of the 13th game of the season.

Guimaraes is one of the 10 venues being used for this year's European Championship in Portugal.