MotoGP star Valentino Rossi led the tributes for Marco Simoncelli after he became the second high-profile death in motorsport in a week.
Italian Simoncelli was killed in a crash at the Malaysian Grand Prix on Sunday - exactly a week after British star Dan Wheldon died at the IndyCar 300 in Las Vegas.
The Gresini Honda rider lost control of his bike on the second lap of the circuit in Sepang and was hit by Colin Edwards and then Rossi as he slid across the track.
The 24-year-old's helmet came off in the collision and he was taken to the medical centre for treatment, but died from his injuries.
Simoncelli suffered 'serious trauma' to the head, neck and chest in the accident on lap two.
The organisers cancelled Sunday's race as soon as the extent of Simoncelli's injuries became apparent.
On Twitter, close friend and seven-time champion Rossi wrote: 'Sic for me was like a youngest brother. So strong on track and so sweet in the normal life. I will miss him a lot.'
Spanish rider Dani Pedrosa had a dispute with fellow factory Honda rider Simoncelli earlier this season, but news of his rival's death left last season's championship runner-up reeling.
Repsol Honda's Pedrosa said: 'In a tragedy like this there is not much to say. I just want to give my condolences to his family and all the people who love him.
'I've been with his father and all we could do was to hug, nothing else matters. It was a fatal accident and everybody in the paddock remains in shock.
'Many times we ourselves forget how dangerous this sport can be and when you lose people on the way nothing has any meaning. It's clear that we all do what we like, what we love, but on days like this nothing matters.'
Newly-crowned MotoGP champion Casey Stoner added: 'I'm so shocked and saddened by the loss of Marco. When things like this happen it reminds you how precious life is, it makes me feel sick inside.
'All I can say is how sorry I am for Marco's whole family. I can't imagine what they are all going through and our thoughts and wishes are with them at this time. I hope they all stay close and pull through this tragedy together.'
Speaking at a press conference following the tragedy, medical director Dr Michele Macchiagodena said of Simoncelli: 'Because of the crash he had during the race, in which he was hit by other riders, he suffered a very serious trauma to the head, to the neck and the chest.
'When our medical staff got to him he was unconscious. In the ambulance because there was a cardiac arrest they started CPR (cardiac pulmonary resuscitation).
'Immediately in the medical centre, with the help also of the doctor of our staff at the Clinica Mobile and local doctors, he was intubated and it was possible to take off some blood from the thorax.
'The CPR was continued for 45 minutes because we tried to help him for as long as we thought it was possible. Unfortunately it was not possible to help him and at 16:56 (local time) we had to declare he was dead.'
Simoncelli's compatriot and Repsol Honda rider Andrea Dovizioso said: 'I watched the images and I'm shocked: in a race you fight and push hard and disaster is often around the corner.
'Marco was a strong rider and he always pushed hard. We raced together since we were kids, I saw him always pushing to the maximum, he crashed many times, but without major injuries, he was seemed invincible. What happened today seems impossible.'
Simoncelli, who won the 250cc world championship in 2008, clinching the crown in Sepang, stepped up to MotoGP in 2010 and he finished eighth overall last season.
'Everybody involved in MotoGP extends its deepest condolences to Marco's family, friends and team at this tragic loss.'
Simoncelli, who won the 250cc world championship in 2008, clinching the crown in Sepang, stepped up to MotoGP in 2010.
A message on Simoncelli's Gresini Honda team website read simply 'CIAO "SUPER SIC"'.
The Rizla Suzuki team wrote: 'All at Rizla Suzuki are deeply affected by today's tragic events and send their deepest condolences to Marco's family and friends during this time of great loss.
'R.I.P Marco Simoncelli, 20 January 1987 - 23 October 2011 - A true competitor.'
Of the other riders involved, Edwards suffered a dislocated shoulder after being thrown from his bike, while Rossi managed to limp back to the pits.
Meanwhile, Rossi's mentor has denied the Italian great will be retiring from the sport in the wake of his Simoncelli's death.
Davide Brivio, Rossi's manager at former team Yamaha, responded to widespread internet rumours that the 32-year-old was too upset to carry on racing.
'Many are asking about Vale stopping races. This is absolutely not true and I'm sorry even that I have to explain it,' Brivio wrote on Twitter.