ISLAMABAD — The 24-year-old son of former Pakistani Prime Minister Benazir Bhutto launched his political career on Thursday with a fiery speech before thousands of cheering supporters observing the fifth anniversary of his mother’s assassination.

Bilawal Bhutto Zardari’s speech comes several months before national elections are expected to be held. He is too young to participate in the elections himself — the minimum age is 25 — but probably will be a key asset for the ruling Pakistan People’s Party. The party’s popularity has plummeted since it took power nearly five years ago as the country has struggled with a weak economy and bloody Taliban insurgency.