Some 9.6 million Afghans are registered to vote but many have lost any hope that after 18 years of war, any leader can unify the fractious country. (AFP)

Afghans head to the polls amid tight security Saturday to vote for their next president, following a bloody, two-month election campaign that has been marred by repeated attacks across Afghanistan.

The Taliban, who already have unleashed a string of bombings, have in recent days issued repeated warnings they intend to attack polling centers and have told voters to stay away.

Wary authorities have placed an uneasy Kabul under partial lockdown, banning trucks from entering the city in an effort to stop would-be suicide bombers attacking the electoral process.

The capital’s traffic, often gridlocked, has dropped to a trickle as schools and offices close for the day and as many people choose to stay off the roads.

Some 9.6 million Afghans are registered to vote, but many have lost any hope that after 18 years of war any leader can unify the fractious country and improve basic living conditions, boost the stagnating economy or bolster security.

Several of the 18 original contenders have dropped out and the poll is considered a two-horse race between current President Ashraf Ghani and Abdullah Abdullah, the country’s chief executive.

Both claimed victory in the 2014 election — a vote so tainted by fraud and violence that it led to a constitutional crisis and forced then-US president Barack Obama to push for a compromise that saw Abdullah awarded the subordinate role.

Voting in Afghanistan’s fourth presidential election began 7 am at some 5,000 polling centres across the country. Authorities had initially planned to open hundreds more but were unable to owing to the abysmal security situation.

Campaigning was hampered by violence from the first day, when Ghani’s running mate was targeted in a bomb-and-gun attack that left at least 20 dead.

Bloody attacks have continued to rock Afghanistan, including a Taliban bombing at a Ghani rally last week that killed at least 26 people in the central province of Parwan near Kabul.

The interior ministry says 72,000 forces will help to secure polling stations.

Stephane Dujarric, spokesman for UN Secretary-General Antonio Guterres, said: “any acts of violence against the electoral process, including attacks directed at polling centres, polling staff and voters, are unacceptable.”

Election officials say this will be the cleanest election yet held, with equipment such as biometric fingerprint readers and better training for poll workers to ensure the vote is fair.

Still, the US embassy in Kabul has said it is “disturbed by so many complaints about security, lack of an equal playing field and fraud” and many Afghans say they have no intention of voting, citing fraud and security fears.

Saturday’s poll was initially slated to take place in April, but was twice delayed because election workers were ill prepared, and the US was leading a push to forge a withdrawal agreement with the Taliban.

That deal has been scuppered for now after US President Donald Trump pulled out.