Both the PPP and PML-N have complained of widespread rigging by allies of Pervez Musharraf, the Pakistani president, and have threatened to launch protests if they are robbed of victory.
Musharraf, who retired from the army in November, rejects complaints of rigging and says procedures have been refined to prevent cheating.
Pakistan People's Party: Late Benazir Bhutto's party is now led by her widower Asif Ali Zardari and son Bilawal Bhutto Zardari.
An arrow is its symbol on the ballot.
Pakistan Muslim League-N: Led by two-time prime minister Nawaz Sharif, the party is fiercely opposed to Musharraf. The tiger
is its symbol on the ballot.
Pakistan Muslim League-Q: The ruling party after the 2002 poll, it supports Musharraf.
A bicycle is its symbol on the ballot.
He says he is ready to work with whichever party forms a government and chooses a prime minister.
In all, 106 parties are contesting, of which 15 were represented in the last parliament.
In 2002 the governing PML-Q won 130 seats and three allied parties won another 53 seats.
The PPP had 63 seats; an alliance of six conservative Muslim parties called the Muttahida Majlis-e-Amal or United Action Front won 59, and the PML-N won 18.
This time around, the Muttahida Majlis-e-Amal is expected to fare badly, although this has more to do with its failure to bring improvement in NWFP, analysts say.
Smaller parties, including that of Imran Khan, nationalist groupings and independents won 19.
Polling stations across Pakistan open on Monday at 8am local time (0300 GMT) and close at 5pm (1200).
Ballots will be counted in polling stations where they are cast and results are expected to start coming out towards midnight on Monday.
The trend in the battleground province of Punjab, where half the members of parliament will be elected, should be clear late on Tuesday morning, with unofficial results out later in the day.
Saturday's biggest attack in the NWFP occurred at a rally of the PPP in the tribal town of Parachinar, which lies opposite the Afghan area of Tora Bora.