The Lions had appeared headed to their first Super Bowl appearance after building a 24-7 half-time lead in the NFC title clash in San Francisco, but the 49ers needed just eight minutes in a stunning third-quarter comeback to erase a 17-point deficit before grinding out a 34-31 win.

Detroit looked poised to run the Niners out of their own stadium when, on just the fourth play from scrimmage, Jameson Williams slashed his way to a 42-yard touchdown and David Montgomery and Jahmyr Gibbs rushed for two more as the Lions charged ahead 24-7.

However, a refocused Niners stormed out after the break to wipe out Detroit's advantage on a 43-yard Jake Moody field goal and a six-yard touchdown pass from Brock Purdy to Brandon Alyuk before the NFL's leading rusher Christian McCaffrey ploughed over from the one-yard line for his second score of the game.

A 33-yard Moody field goal in the fourth quarter gave the 49ers their first lead of the game at 27-24 before Elijah Mitchell added a three-yard touchdown to extend their advantage.

Detroit did not go down without a fight with Williams hauling in a touchdown pass from Jared Goff with 56 seconds left to play but the Lions could not recover the onside kick, ending the late rally.

"We played as bad as we could in the first half," said 49ers coach Kyle Shanahan.

"There's been unfinished business for a while our team has been set up for this for a long time it's been a long year to get to this point and we got it done today.

"It was hard at the beginning but the character we have on our team, the type of guys we have, can't wait to get to Vegas."