The flames were quickly put out as the classic finished with a goalless draw, observed from the Dragons bench by Real Madrid legend Pepe.
In the Primeira Liga classic, Porto were fighting to extend their seven-point league over second-placed Benfica – who were reportedly rejected by Jose Mourinho after their approach over a coaching role.
Enthusiastic Sporting supporters, whose club are still in race for a Champions Leagues spot, prepared an impressive display to greet their pupils coming onto the pitch.
Lions had a jam-packed house at Alvalade with supporters waving flags with the club's crest, throwing paper tapes onto the turf, and firing flares.
However, the fans appeared to be a tad overzealous as one of the tapes caught fire in one of the blocks.
The fire then spread to the chairs in bizarre scenes before the referee's whistle.
But the flames were fortunately put out quickly with no greater damage caused.
The match finished with a goalless draw and Porto, led by Manchester United target Eder Militao, saw their lead over Benfica trimmed to five points.
It was also a 100th appearance in the Primeira Liga for Sporting's captain, and a former Red Devils player, Nani.
Real Madrid legend Pepe sat on the bench for Dragons, returning to the club after 12 years.
The 35-year-old defender was linked with a move to Thierry Henry's Monaco, but eventually signed a contract with Porto tying him down until 2021.
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