The family of Motherwell player Phil O'Donnell, who died after collapsing during a match on Saturday, said yesterday they had been overwhelmed by the support and messages from thousands of people around the world.

The Motherwell FC website crashed at one stage as football fans paid tribute to the 35-year-old club captain. Scarves, strips, flags, flowers and pictures swamped the gates at the Fir Park ground where O'Donnell collapsed during a Premier League fixture against Dundee United, which Motherwell went on to win 5-3.

A postmortem is expected today.

O'Donnell, who was married with four children, was the latest of several professionals and promising youth footballers around the world to die during matches or while training.

Fifa, the international governing body of football, said in August they were considering introducing cardiac testing of players.

Alex Salmond, Scotland's first minister, was among those to pay tribute to a popular sportsman whose once promising career had been blighted by injury.

Bernard O'Donnell, Paul's father, said in a statement on behalf of the family: "None of us can yet comprehend what has happened but we have been overwhelmed by the support and messages from people inside and outside the football world.

"We are very grateful for all the support we have received. In the meantime, we would appreciate it if our privacy is respected and we get time to grieve."

O'Donnell was rushed to hospital after apparently suffering a seizure, but he did not regain consciousness.

His nephew David Clarkson, also a Motherwell player, had scored two goals and was on the pitch when O'Donnell collapsed in the 72nd minute as he was substituted.

Clarkson was quickly substituted himself and accompanied his uncle to hospital.

Lee Wilkie, a Dundee United defender who put O'Donnell into the recovery position, said: "I was a few yards away and I just saw him collapse. Someone said he had swallowed his tongue, but I went and checked and his tongue was there. But his eyes were wide open, and it was obvious something was really wrong."

Motherwell striker Chris Porter said: "He went down and no one really knew what had happened. Doctors and physios ran on, he was unconscious on the floor and it was clear he was in a bad way. I'm just really shocked by it all."

Mark McGhee, Motherwell's manager, said players, fans and club officials were "devastated". Team-mates had "taken it very badly and we're going to have to help them through that". He added: "We're going to miss a guy who was a fine man, a guy who was a fantastic example to all of his colleagues. He was a man amongst boys in every sense of the word in his attitude and professionalism and integrity."

O'Donnell made his debut for Motherwell as a 17-year-old and made just one international appearance for Scotland, against Switzerland.

He moved to Celtic in the 1994-95 season, playing over 100 games in five years, before moving to Sheffield Wednesday. He then returned to Motherwell.