Kansas City Chiefs defence of their Super Bowl crown got off to a shaky start as they went down in a mistake-ridden 21-20 defeat to the Detroit Lions in the NFL curtain raiser on Thursday.

The Chiefs, who have been to three of the last four Super Bowls and have won twice, headed into the new NFL season as favourites to retain their crown.

But they slipped to their first opening week loss in nine games as the Lions running back went in for the crucial score with little more than five minutes remaining.

The Chiefs did have one last chance when Lions quarterback Jared Goff had a fourth down pass batted down near midfield, but two dropped passes and two penalties left Patrick Mahomes with just a heave downfield in the last minute which fell incomplete.

Despite his receivers dropping a string of passes, Mahomes threw for 226 yards with two touchdowns, as well as finishing as the leading rusher for the Chiefs.

But he also had a pass intercepted by rookie Brian Branch which was returned for a touchdown from 50 yards.

Goff threw for 253 yards and a touchdown pass to Amon-Ra St Brown, who opened the campaign with six catches for 71 yards.

The Chiefs began the season without star tight end Travis Kelce, who injured his knee in practice on Tuesday, and fellow All-Pro Chris Jones who is in a contract dispute.