Vast crowds gathered Thursday to mark the fifth anniversary of the assassination of former Pakistan premier Benazir Bhutto, and to witness her son launch his own political career.More than 200,000 people gathered at the family mausoleum in Garhi Dera Bakhsh in the southern province of Sindh to pay their respects and to hear Bilawal Bhutto Zardari, the son of Benazir and of President Asif Ali Zardari, make his first major public speech.Bhutto, twice elected prime minister, was killed in a gun and suicide attack after an election rally in Rawalpindi, the headquarters of Pakistan's army, on December 27, 2007. No one has ever been convicted of her murder.Security was tight around a huge stage, adorned with the red, black and green tricolour of the ruling Pakistan People's Party (PPP), where Oxford-educated Bilawal will speak.Surveillance helicopters hovered overhead as police commandos stood alert and sniffer dogs searched for explosives. Police said more than 15,000 officers had been deployed, as well as some 500 government paramilitary forces.The Bhutto family has been a force in Pakistani politics for almost all of the country's 65-year history. Benazir's father Zulfikar Ali Bhutto, who founded the PPP, led the country from 1971 until he was ousted in a military coup in 1977. He was hanged in 1979 after being convicted of authorising the murder of a political opponent.Bilawal was just 19 when his mother was killed, and his spokesman Aijaz Durrani said Thursday's anniversary would mark the start of a new chapter in Pakistan's political history."This is his political career's first public meeting. A new Bhutto is emerging today in the shape of Bilawal who has vision of his mother and grandfather and people are excited on his launching," he told AFP.As head of state President Zardari, who came to power in elections held a month after his wife's murder, is barred from leading the PPP election campaign. He is also hugely unpopular, tainted by years of corruption allegations.A general election is due in the spring and though the 24-year-old Bilawal will be too young to stand -- the lower age limit is 25 -- political analyst Hasan Askari said he could provide a fresh new figurehead for the PPP campaign."Bilawal has symbolic value in the Bhutto family and Zardari would like this link to be used as symbolism in the election," Askari told AFP.The Bhuttos are an almost ever-present element in the rhetoric of PPP leaders, who frequently eulogise the party's two "martyrs" as champions of the common man's struggle against a repressive "establishment".Prime Minister Raja Pervez Ashraf said the country should "shun prejudices and maintain unity" to pay homage to Benazir."Let us resolve to defeat the forces of extremism and terrorism and work for the progress and prosperity of the country," he said in a statement.Bilawal, who has been co-chairman of the PPP with his father since Benazir's death, in May accused former military ruler Pervez Musharraf of "murdering" his mother by deliberately sabotaging her security.A UN report in 2010 also said the murder could have been prevented and accused Musharraf's government of failing to protect Bhutto properly.The Musharraf regime blamed the assassination on Pakistani Taliban chief Baitullah Mehsud, who denied any involvement and was killed in a US drone attack in August 2009.There has been a surge in terror attacks in Pakistan in recent weeks. Brigadier Saad Khan, a former officer with the powerful Inter-Services Intelligence agency, warned the Taliban may continue their campaign with an attack on events marking the anniversary.