The 24-year-old son of former Pakistani Prime Minister Benazir Bhutto launched his political career 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 is likely to 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.

Before dawn on the same day, dozens of militants armed with rocket-propelled grenades and automatic weapons attacked two tribal police posts in Pakistan's northwest, killing two policemen, officials said. Twenty-one other policemen are missing and presumed kidnapped.

Zardari was made chairman of the Pakistan People's Party after his mother's death but has mainly played a background role until now while he completed his studies at Oxford University in Britain.

"I want to tell you that thanks to God he has completed his studies, but now is the time of his training," his father, President Asif Ali Zardari, told the crowd of supporters Thursday in Garhi Khuda Bakhsh village in southern Sindh province, site of the Bhutto family mausoleum. "He has to study Pakistan, he has to learn from you and he has to work according to your thinking,"

The Bhutto family has played a prominent role in Pakistani politics for much of the country's 65-year history. Bilawal Bhutto Zardari's grandfather, Zulfikar Ali Bhutto, founded the Pakistan People's Party and served as both the country's president and prime minister in the 1970s. He was eventually hanged in 1979 after Gen. Zia ul-Haq seized power in a military coup.

Benazir Bhutto twice served as prime minister in the 1980s and 1990s but never completed a full term. Her governments were dismissed both times under the cloud of corruption allegations by presidents who were close to the country's powerful army. She was killed in a gun and suicide bomb attack on Dec. 27, 2007, shortly after returning from self-imposed exile to participate in national elections.

After her death, the Pakistan People's Party rode a wave of public sympathy to garner the most seats in the 2008 elections, and Asif Ali Zardari was elected president. But the popularity of both the party and the president has fallen significantly since then as the government has failed to address pressing problems, such as Pakistan's shortage of electricity and stuttering economy. The government has also struggled in its fight against the Pakistani Taliban, who have killed thousands of people in attacks throughout the country.

Rasul Bakhsh Rais, a political science professor at Lahore University of Management Sciences, said it was not a surprise that the Pakistan People's Party unveiled Bilawal Bhutto Zardari in an attempt to boost its fortunes in the upcoming elections, which are expected by June at the latest.

"This is Pakistan and dynastic politics is the norm," said Rais. "Bilawal is perhaps the only card left in the chest of the Pakistan People's Party."

Both Bilawal Bhutto Zardari and his father sought to whip up the emotions of the crowd Thursday by shouting "Long live Bhutto" and "Bhutto is alive." Many of the supporters waved the red, black and green flag of the Pakistan People's Party and held pictures of Benazir Bhutto and her father.

"If you kill one Bhutto, one thousand more Bhuttos will emerge," said Bilawal Bhutto Zardari.

He took a swipe at the judiciary, which has clashed with the current government, by asking why people arrested for suspected involvement in his mother's murder have yet to be convicted.

But some critics have questioned why Zardari has not done more to push forward the investigation during more than four years as president.

The president at the time of her death, Gen. Pervez Musharraf, blamed the Pakistani Taliban for the attack, and five suspected militants are facing trial for alleged involvement in the killing. The Pakistani Taliban have denied targeting Bhutto.

A Pakistani court issued an arrest warrant for Musharraf last year over allegations he played a role in the attack, which he has denied. Arrest warrants were also issued for two senior police officials accused of negligence in the assassination. Prosecutors accused one of the officials of failing to provide proper security for Bhutto and the other of cleaning the crime scene before evidence could be collected.

A U.N. investigation into the assassination said it could have been prevented and blamed all levels of government for failing to provide adequate security. It also accused intelligence agencies and other officials of severely hampering the investigation into those behind her murder.

The attack on the tribal police posts before dawn Thursday took place in the town of Darra Adam Khel in Khyber Pakhtunkhwa province, government officials said. The town is located near Pakistan's tribal region, the main sanctuary for Taliban militants in the country.

Security forces have launched an operation to try to recover the 21 missing policemen, said the officials, speaking on condition of anonymity because they were not authorized to talk to the media.

No group has claimed responsibility for the attack, but suspicion will likely fall on the Pakistani Taliban.

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Associated Press writer Riaz Khan contributed to this report from Peshawar, Pakistan.