ISLAMABAD — Defying threats of violence, Pakistanis streamed to the polls Saturday for a historic vote pitting a former cricket star against a two-time prime minister and an unpopular incumbent. But militant attacks that killed 20 people underlined the risks many people took just casting their ballots.

The violence was a continuation of what has been a bloody election season, with more than 130 people killed in bombings and shootings. Some are calling this one of the deadliest votes in the country’s history.

Despite the violence, many see the election — the country’s first transition between an elected government fulfilling its term to another — as a key step to solidify civilian rule in a country that has experienced three military coups.

With the Pakistani Taliban threatening to target political parties in the vote, the government deployed 600,000 security personnel across the country to protect voters.

The secretary of the election commission, Ahmed Khan, said that he expected the turnout to be “massive.”

This vote is notable for more than just the historic handoff of power from one civilian government to another.

The rise of former cricket star Imran Khan has reshaped the Pakistani political scene, challenging the stranglehold of the country’s two main parties and making the outcome of the vote very hard to call.

The 60-year-old Khan is facing off against the Pakistan Muslim League-N, headed by two-time former prime minister Nawaz Sharif and the Pakistan People’s Party, led by President Asif Ali Zardari.