Violence has erupted as millions head to the polls in Pakistan, with the worst incident seeing at least 27 killed by a bomb in the city of Quetta.

Elsewhere, minor blasts and clashes between party workers left several injured and one dead.

Voters are deciding between the parties of the former cricket star Imran Khan and the disgraced former Prime Minister Nawaz Sharif.

But the campaign has been overshadowed by concerns of fraud and violence.

The Human Rights Commission of Pakistan says there have been "blatant" attempts to manipulate the polls.

Despite tight security across the country, with more than 370,000 troops deployed to secure the ballot, violence has broken through.

Officials say the attack in Quetta, in the restive province of Balochistan, was a suicide bomb targeting police at the gate of a polling station. An attack earlier this month in nearby Mastung killed at least 149 people in one of Pakistan's deadliest-ever suicide bombings. It was claimed by the Islamic State group.

Pakistan is no stranger to political turmoil and the last few months have proved no exception. Nawaz Sharif, the man who won the last election, is watching this contest from prison. He has been jailed for corruption after a scandal stemming from the Panama Papers leak.

In an interview with BBC Urdu on Monday, the former prime minister's daughter Maryam Nawaz - who was jailed earlier this month with her father on related charges - criticised the all-powerful military.

"When a prime minister refuses to put down his head and do their [the military's] bidding, they pull him down with four things; get a religious fatwa issued against him, call him a traitor, call him a friend of India, or call him corrupt. They use these things against every elected prime minister," she said.

Women in Pakistan have the right to vote - however many who live in socially conservative areas are pressured into not voting.

Authorities are trying to change this with new rules stipulating that at least 10% of voters in each constituency must be women in order for the results to be valid.

What's the context for this vote?

Pakistan has been ruled on and off by the military during its 71-year history. This election is significant because it will mark only the second time that one civilian government has handed power to another after serving a full term.

But the run-up to the vote has been controversial.

The Pakistan Muslim League-Nawaz (PML-N) complains of a targeted crackdown by the powerful security establishment, with the alleged help of the courts, in favour of Imran Khan and his Pakistan Tehreek-e-Insaf (PTI).

On Sunday, a judge in the High Court of Islamabad appeared to back up that allegation, saying that the military Inter-Services Intelligence (ISI) organisation had been interfering in the judiciary.

Several PML-N candidates also say they have been coerced to switch to the PTI, and nearly 17,000 party members are facing criminal cases over breaking unspecified election rules. The Pakistani military denies interfering in politics.

Independent media, meanwhile, say there have been blatant attempts to muzzle them. There are also serious concerns about the participation of internationally designated militants in the election process.

For all these reasons, the human rights commission has said there are "ample grounds" to question the legitimacy of the polls, "with alarming implications for Pakistan's transition to an effective democracy".

Has Imran Khan's time come?

As voters queue up outside polling stations, the question on everyone's mind will be whether Mr Khan will be the next prime minister.

He entered politics in the late 1990s when memories of Pakistan's cricketers winning the World Cup were still fresh. But only in 2013 did his party emerge as a serious contender, with its leader touted as a harbinger of change out to fight corruption.

He lost by a wide margin, but has continued to lead what many see as a divisive campaign.

This election is the closest he's ever got to being prime minister. Will that happen? Or will the PML-N stay the largest party after a sympathy wave by voters for the man he helped oust?

Who are the main candidates?

The election is generally seen as a contest between the PML-N and the PTI.

Nawaz Sharif - a 68-year-old three-time PM - was convicted by an anti-corruption court and sentenced in absentia to 10 years in prison. He was disqualified from office last year by the Supreme Court after the Panama Papers leak revealed his family's ownership of several luxury flats in London.

Sharif returned from London earlier this month with his daughter, Maryam, and both were arrested. Their appeal hearings will not be held until after the vote.

Sharif's brother, the former governor of Punjab province, Shehbaz Sharif, 66, is now leading the party.

But Imran Khan and his PTI have been rising in the polls. Mr Khan, 65, told the BBC last week that his opponents will "lose because of their track record" when in power.

The country's best-known opposition politician, who is running on an anti-corruption ticket, has denied colluding with the military. Analysts say he will have to make serious inroads in Punjab province - a PML-N stronghold - in order to win the vote.

The party of assassinated former prime minister Benazir Bhutto, the PPP, is widely expected to come third.

Now fronted by Ms Bhutto's son, Bilawal Bhutto Zardari, a 29-year-old Oxford University graduate, the historically liberal party is promising to create a "peaceful, progressive, prosperous, democratic Pakistan".