The Pakistan military will station over 370,000 troops nationwide to ensure the vote goes smoothly, the largest such deployment in the country's history on an election day.

The Election Commission of Pakistan (ECP) later said military officers would also be given magisterial powers, effectively making them judge and jury to punish individuals for illegal acts committed inside polling stations.

"I don't know why they have given these powers, because that will unnecessarily create doubts in the minds of people," retired general and security analyst Talat Masood told AFP.

"I don't think these powers have ever been granted."

Election observers also questioned the move, and said there was rising anxiety over the large military presence at the polls.

"A lot of our interlocutors, and I would dare to say most of them, they raise serious concerns regarding the role of the military," said Dimitra Ioannou, deputy chief observer of the European Union Election Observation Mission.

Last week, Sherry Rehman -- opposition leader in the Senate, the parliament's upper house -- said the move could lead to potential conflicts and confusion. Raza Rabbani, another high-profile senator, demanded a clarification from the ECP.

The ECP said Sunday the presence of troops at polling stations is meant to ensure a "free and fair election".

The military -- which has ruled the country for roughly half its 70 year history -- remains Pakistan's most powerful institution and has a long history of meddling in politics and judicial affairs -- a charge it denies.

"It would be difficult to call the elections free and fair," Ibn Abdur Rehman of the Human Rights Commission of Pakistan told AFP Monday, following a press conference on media censorship during the campaign season.

The controversy comes as increasing militant attacks on campaign events in the last month have raised fears that insurgents may target voters.

Three candidates have been killed in attacks at political events this month, including a member of cricketer-turned-politician Imran Khan's Pakistan Tehreek-e-Insaf (PTI) party in a suicide attack on Sunday.

And on Monday, authorities announced an increased death toll -- 153 -- for an earlier attack on a rally in the town of Mastung in southwestern Balochistan province, making it the second-deadliest terror attack in Pakistan's history.

The increasingly bitter contest is expected to be a tight race between jailed former prime minister Nawaz Sharif's incumbent Pakistan Muslim League-Nawaz party and Khan's PTI.