Britain's Mo Farah missed out on 10,000m gold in agonising fashion as he was overtaken by Ethiopia's Ibrahim Jeilan just metres from the line.
A gutsy run saw Farah become the first Briton to win a world medal at 10,000m.
But it had looked like being gold and not silver as the 28-year-old stormed to the front with less than two laps remaining of a thrilling 25-lap race.
Farah told BBC Radio 5 Live: "At the end I just didn't have anything left. It's agony. You always want the gold."
Farah, the fastest man in the world over 10,000m this year, kept pace with the leaders throughout and looked comfortable as he kicked for home with around 600 metres left.
But Jeilan slowly clawed back Farah's healthy advantage and overtook the Briton as they entered the home straight to win in a time of 27 minutes 13.81 seconds, with Farah just 0.26secs behind.
Jeilan's fellow Ethiopian Imane Merga claimed the bronze but Kenenisa Bekele, winner of the last four 10,000m titles, dropped out with 10 laps still to go.
Three-time Olympic champion Bekele had been sidelined since January 2010 through injury and never looked likely to challenge for a medal.
Farah had said he would be battling his nerves but showed no signs of anxiety as he cruised into contention. The pace varied during the early stages but he made sure he kept in touch.
It appeared as though Farah had got his tactics spot on as he powered to the front in the closing stages, only for Jeilan to move ahead with 35 metres left.
However Farah, who has made huge improvements since moving with his family to Oregon at the start of the year to be coached by Alberto Salazar, could not hide his disappointment
He became only the second British global medal winner over 5,000m following a bronze by Jack Buckner in 1987.
But Farah, who also won Britain's first medal at this year's World Championships, said: "You give it 110% and it's not enough sometimes.
"I thought I'd got it right but it wasn't enough. I'll have a chat with my coach, analyse it and see what happened. It's nice to win a medal but I am disappointed with silver."
And Farah denied he burst for home too early.
"I always wanted to go at 400, 500m," he insisted. "That's my best tactics. I thought I had that speed at the end but he was finishing quicker.
"I thought to myself if I could run 52 or 53 seconds (for the last lap) that would be enough. But it wasn't enough."
"It means a lot winning a major medal. It would have been nicer with a gold but the better man won on the day and fair credit to him.
"I'll see how I go from here, hopefully see what I can do in the 5k."
The heats of the 5,000m are on Thursday, with the final on Sunday.
Farah has been in superb form this season, stunning a top-class field to set a new British and European 10,000m record of 26 minutes 46.57 seconds at the Diamond League meeting in June.