Twitter was awash with tributes today after the tragic death of fitness instructor, model and actor Greg Plitt over the weekend.

Greg was hit by a train on Saturday while filming an ad for one of his diet products in Los Angeles.

"He had on all black. The train went by. I saw him stumble over the tracks. He had a camcorder in his hand," said one onlooker.

The renowned fitness instructor is also known for his modelling work with Calvin Klein and his acting roles in The Good Shepherd with Matt Damon and Terminator 4 with Christian Bale. 

Many celebrities have taken to social media to share messages of sadness and shock, with glamour model Jodie Marsh leading the tributes. "Holy s**t, just seen the awful news about greg Plitt. An amazing fitness model & so hugely motivational. He will be so missed #RIPGregPlitt," she wrote on her Twitter page this morning.

Celebrity personal trainer Russ Howe also expressed his disbelief at the shocking accident. "R.I.P. Greg Plitt. One of the most awesome people the fitness industry has ever known," he wrote.

Our thoughts are with Greg's family at this hard time.