THE world of sport was in mourning last night after Formula One driver Ayrton Senna died following a horrendous crash at the Italian track of Imola. The 34-year-old Brazilian, a world champion three times, was pronounced clinically dead four hours after being admitted to hospital. He was declared dead 25 minutes later. Trackside doctors had to re-start his heart after it stopped beating when they dragged him out of the wreckage of his Williams car. Senna's accident came just 24 hours after Austria's Roland Ratzenberger was killed in practice, and two days after Rubens Barrichello was miraculously spared serious injury when his Jordan Hart disintegrated in a high-speed smash. Ratzenberger was the first man to be killed in Formula One racing since Italy's Ricardo Paletti died in the Canadian Grand Prix in 1982. It was the first time in 35 years two drivers had been killed in a Grand Prix weekend. Senna was leading the incident-packed race as he went into the Tamburello turn at about 350 km/h. His Williams-Renault suddenly lost all grip and the car careered off the circuit and slammed nose-first into a concrete wall. The 34-year-old Brazilian was quickly lifted from the wreckage, but lay for a quarter of an hour on the trackside as medics gave him first aid. A large pool of blood marked the spot where he had been lying when Senna was eventually stretchered into the helicopter. ''He looked nervous from the very first lap,'' said eventual winner German driver Michael Schumacher, who was behind Senna when the accident occurred. ''He took two or three bumps, but I can't say what happened exactly.'' Senna was shocked by Ratzenberger's crash and did not drive when qualifying was resumed Saturday. He retained the pole position - for a world record 65th time - with his time from Friday's session. Senna, whose brilliance on the track was matched only by his fiery reputation, won 41 Grands Prix in his career, second only to Alain Prost. Senna won the world title with Team McLaren in 1988, '89 and '91. ''To survive in Grand Prix racing you need to be afraid,'' Senna once said. ''Fear is a very important feeling to have. It helps you stay together. It helps you race longer . . . and live longer.'' It was the third horrific accident in as many days on the high-speed Enzo and Dino Ferrari circuit, but only one of three to cause injury yesterday. Finnish driver J. J. Lehto, back in Formula One after a pre-season practice smash in January, stalled his Benetton Ford and was rammed from behind by Pedro Lamy in the Lotus Mugen Honda, spraying debris in all directions. Three spectators and one police officer were injured. The race was almost immediately suspended, with the pace car guiding drivers round for the first five laps. The race had only just re-started when Senna veered off at the same place where Nelson Piquet, Brazil's other triple world champion, had crashed in 1987 and near where Ratzenberger met his death on Saturday. Surprisingly, organisers decided to re-start the race. Schumacher swept to his third consecutive victory, while Nicola Larini gave Ferrari a rare second place and Finland's Mika Hakkinen earned McLaren-Peugeot their first podium. Yet there was more drama just minutes from the end when Michele Alboreto's Minardi lost a wheel and ploughed into a group of mechanics in the pits. Three of them were injured - two from Ferrari and one from Lotus. The extent of their injuries was not known. The weekend's crashes prompted calls for changes from both Schumacher and France's former world champion, Alain Prost. ''I can't feel satisfied. I can't feel happy,'' Schumacher said afterwards. ''It was not just one thing but so many things. The only thing I can say is there's a lot to learn from it. ''What happened this weekend was so dramatic, so bad,'' he added. ''It's not right what happened.'' Prost, who retired last season after winning his fourth world title, said: ''When I talked about safety, about the dangers of driving in rain I was called a coward. This weekend has shown that safety is a major issue in Formula One.'' Jackie Stewart believed the drivers should form a drivers' body to make Formula One a safer sport. ''Ayrton Senna was a great driver, and this is really awful. I think the drivers should get together as in my time with the Grand Prix Drivers Association.'' The Scottish three-time world champion continued: ''They will then be able represent themselves against sports authorities, and, united, they could refuse to drive on a bad circuit. What happened at Imola during the weekend showed the circuit was dangerous. ''At Imola, you can go 150 mph (250km/h) everywhere and when you spin off, you hit a wall.'' Senna had been under pressure to deliver a victory, having failed to finish the first two races of the season, the Brazilian and Pacific Grand Prix. Senna was even being tipped as race favourite on the Imola circuit, known to be one of the fastest in the world championship, as it suits the more powerfully engined cars such as the Williams. Senna was one of only seven drivers to win the world championship three times. Acknowledged as one of the most accomplished drivers ever, the determined but temperamental Brazilian had set his sights on a fourth championship this season. But the 34-year-old Williams driver, runner-up to Frenchman Prost last season, had not started this year well, failing to finish the first two races. Senna's career included three wins at Imola's Autodromo Enzo e Dino Ferrari, in 1988, 1989 and 1991. He first became Formula One world champion in 1988 when he won eight of the 16 races, a record for one season, driving with McLaren. Senna took the championship again in 1990 and 1991, making him the youngest man to win three world titles. By the end of last year, when he switched from McLaren to Williams, Senna had won 41 Grands Prix from 161 starts. Yesterday's race was his 65th start from pole position. ''Winning is like a drug,'' he once said. ''I cannot justify in any circumstances coming second or third.''