Pakistan's Taliban has claimed that it is planning to carry out suicide bombings on Saturday's election.
The move is an attempt to undermine the poll, according to a letter from the head of the organisation.
The letter calls for attacks in several parts of the country to disrupt "the system of infidels which is called democracy".
Pakistan's military said it will send tens of thousands of troops to polling stations and counting centres to prevent the Taliban's disruption.
Since April, the al-Qaeda-linked Pakistani Taliban has killed more than 100 people in attacks on candidates and rallies.
It has targeted particularly those of secular-leaning parties, in an attempt to undermine elections they regard as un-Islamic.
The attacks have prevented candidates from the three main parties in the ruling coalition from holding big rallies.
Instead, they have relied on door-to-door campaigning or small meetings in homes or on street corners.
The election, already Pakistan's most violent, marks the first time that a civilian government will complete a full term and hand over to another administration.
Army spokesman Major General Asim Bajwa said 300,000 security officials, including 32,000 troops, have been deployed in Punjab, the most populous province.
Another 96,000 security forces will be deployed in the northwest of Pakistan, where the Taliban operates from strongholds.
However, the militants have not attacked the main opposition party, led by former Prime Minister Nawaz Sharif, which has courted support from groups accused of supporting militancy.
Mr Sharif, who is seen as favourite to become the next prime minister, says Pakistan should reconsider its support for the US war on Islamist militancy.
He suggests he would be in favour of negotiations with the Taliban.
The Taliban has not attacked former cricketer Imran Khan's party either.
It advocates shooting down US drones and withdrawing the Pakistani military from ethnic Pashtun areas along the Afghan border.