Islamabad - Pakistani lawmakers Saturday were casting their votes to elect the successor to former president Pervez Musharraf who resigned last month to avoid impeachment in the parliament.

Asif Ali Zardari, widower of former prime minister Benazir Bhutto who heads her Pakistan People's Party (PPP) since her assassination last year, is expected to win the presidential race against the two other candidates.

The secret ballot by both houses of the parliament - the 342-member National Assembly and the 100-seat Senate - and four provincial assemblies which jointly form the presidential electoral college, started simultaneously at around 10 am (0400 GMT) and will continue until 3pm (0900 GMT).

A presidential candidate, who is obliged to be a Muslim, is elected not through a general vote but a complicated system of voting.

Altogether, 1,170 legislatives cast their ballots but not each vote has the same value. Each member of the two houses of the parliament has one vote and so does the every lawmaker of the assembly in the smallest province of Balochistan, which has 65 seats.

In total, the electoral college has 702 votes of which the one who gets the maximum votes will be declared as the winner. Zardari's supporters predicted he will get more than 450 votes.

His closest contender is a former chief justice Saeeduz Zaman Sidduqui, who was nominated by Zardari's former coalition partner and the head of second largest Pakistan Muslim League-Nawaz party, Nawaz Sharif

Both formed an alliance after February 18 general elections to form the government and managed to oust their common enemy Musharraf.

But Sharif broke down the coalition when Zardari refused to restore the senior judges sacked by Musharraf late last year and surrender the presidential powers to dissolve the parliament before nominating himself as the candidate for president's office.

Sharif says those presidential powers could allow Zardari to turn into a civilian dictator, and dash the nation's hopes for supremacy of the parliament.

Although Siddiqui is an old Sharif loyalist, he has standing as a man of principle because as a chief justice he refused to bow when Musharraf took over in a military coup in 1999, and preferred to resign from the post.

He has promised to return the country to parliamentary democracy and reinstate the removed judges.

Third candidate is Mushahid Hussain Sayed, a career journalist and an outspoken politician, who has been nominated by Musharraf's former political ally Pakistan Muslim League-Quaid.

Sayed has publicly raised questions about the mental health of Zardari citing a Financial Times report that Bhutto's widower was suffering from dementia and stress because of his prolonged imprisonment over charges of corruption.

Zardari denies the allegations, and notes that he was never formally convicted of the charges in any court of law.

Indifferent to criticism and even his unpopular ranking among the public, Zardari is poised to win.

The most important challenge he would face as president would be the problem of rising Islamic militancy in the lawless tribal areas, which is a grave threat to the US-led international forces in Afghanistan.

Washington has long been concerned over Pakistan's inability to stop cross-border raids by Taliban.

On Wednesday US troops carried out the first ground attack in the tribal district of South Waziristan in which, according to Pakistani claims, 20 civilians were killed.

The action fuelled anger in Pakistan. Zardari will have to strike a balance between the predominantly anti-Western public sentiment, and the country's need to maintain good ties with the US in its fight against extremism.