Eighty million voters will today decide the political future of Pervez Musharraf, Pakistan's unpopular US-backed president, in a general election that has become a referendum on his eight turbulent years in power.

The results, which are expected to start trickling in around 10pm tonight local time (1900GMT), will determine a new balance of power in a country itching to rid itself of a leader held responsible for a long list of popular grievances, including surging food and electricity prices, growing lawlessness and fast-spreading militant extremism.

Although Mr. Musharraf is not standing, his position as president of the nuclear-armed state may become untenable if the two main opposition parties - the Bhutto family's Pakistan People's party and Nawaz Sharif's Pakistan Muslim League, or PML (N) - sweep the poll and form a hostile government.

"If the PPP and the PML (N) get 70 per cent of the seats, then it may be the end for this system of military rule with a civilian facade," said Sartaj Aziz, a former finance minister and foreign minister under Mr Sharif.

The campaign, marred by bloodshed and overshadowed by the assassination in December of the former prime minister Benazir Bhutto, came to an end amid further violence on Saturday when a suicide blast close to one of her party's offices near the Afghan border killed at least 40 people.

Mr Musharraf, who came to power as army chief in a 1999 coup, secured a second term as president in November only after he temporarily imposed de facto martial law in order to purge the Supreme Court of independent judges who were set to challenge the legality of his re-election.

A PPP/PML(N) coalition government could try to claw constitutional powers back from the president and to restore some or all of the judges dismissed last year, putting itself on a collision course with Mr Musharraf.

Diplomats fear Mr Musharraf, who was forced to step down as army chef late last year, has yet to accept his new quasi-ceremonial role as third fiddle to the new prime minister and to General Ashfaq Kayani, his currently apolitical successor as head of the military.

"His sense that he is the messiah has only grown," said one top western diplomat, who expected "selective rigging" in today's poll. "Musharraf cannot afford to lose, cannot afford to rig massively and yet he cannot win a free and fair election. It's a terrible dilemma."

Polls by organisations such as the International Republican Institute and Gallup Pakistan have indicated that the pro-Musharraf Pakistan Muslim League (Quaid-e-Azam) will suffer a thumping defeat - if the voting is not rigged.

In a fair election, many believe the PPP and PML (N) would be able to oust the PML (Q), dominated by the Chaudhry family, from power in Punjab, Pakistan's wealthiest and most populous province, and might even be in a position to secure the two-thirds majority in parliament needed to impeach the president.

Mr Musharraf, who hopes to divide and rule among the two rival opposition parties and enable the PML (Q) to hold the balance of power in a hung parliament, has -dismissed the polls as a -misleading guide to voter intentions.

Electoral officials say they are watching closely for signs that the PML (Q), with the assistance of the Intelligence Bureau spy agency, police and local officials, is trying to rig the result in 40-50 key marginal seats, mostly in central and southern Punjab. "The desperation of the ruling coalition is so strong because for them it's a matter of survival," said Mr Aziz. "Anywhere you have aggressive candidates from the ruling party, there could be violence."

Saleem Jan Niazi, a fruit seller who voted for pro-Musharraf candidates in the 2002 election, said he had changed his loyalties in the past year, citing rising prices and the government's failure to deliver essential public services. "The electricity doesn't work, the gas doesn't work, inflation is up and in addition to all of those factors Musharraf wants to be president of this country for his entire lifetime," he said. "I am not going to accept that."

The result could highlight divisions in the PPP, which is without a Bhutto at its helm for the first time in 40 years. Some PPP leaders out of power since 1996 want to avoid a confrontation with Mr Musharraf and would be willing to form a government with the PML (Q).

Asif Ali Zardari, Bhutto's widower, has pointedly not excluded working with the retired army chief for a gradual "transition to democracy". But he will struggle to sell this approach to the party's rank and file, many of whom would prefer to join hands with Mr Sharif.