Bahrainis are heading to the polls on Saturday in a parliamentary election from which opposition groups have been barred in a crackdown on dissent in the Western-allied kingdom as tensions with the Shia Muslim opposition show no signs of abating.

Activists have called for a boycott of what they describe as "farce" elections, raising doubts about the credibility of the polls. The government says the elections are democratic.

The polls opened at 8am local time (05:00 GMT) and are set to close at 8pm (17:00 GMT).

Bahrain's Sunni-Muslim ruling Al Khalifa family has kept a lid on dissent since the Shia opposition staged a failed uprising in 2011. Saudi Arabia sent in troops to help crush the unrest in a mark of concern that any power-sharing concession by Bahrain could inspire Saudi Arabia's own Shia minority.

Riyadh regards the neighbouring island nation, which does not possess vast oil wealth like other Gulf states, as a critical ally in its proxy wars with Iran in the Middle East.

Bahrain, which is home to the US Navy's Fifth Fleet, has closed the main opposition groups, barred their members from running in elections and prosecuted scores of people, many described by human rights groups as activists, in mass trials.

"Clearly, legislatures from the world's leading democratic states believe that the upcoming election in Bahrain lacks legitimacy. You simply cannot crush, torture and imprison your entire opposition, call for a pseudo-election, and then demand the respect of the international community," said Sayed Ahmed Alwadaei, director of the UK-based Bahrain Institute for Rights and Democracy (BIRD).

The government said 506 candidates are running in the election, including the highest number of female candidates. It expects a higher voter turnout than in 2014, which it put at 53 percent, when opposition groups boycotted the elections.

Only 23, out of 40 incumbents of the House of Representatives, are seeking re-election this year to parliament, which has limited powers.

Many of Bahrain's Shia say they are deprived of jobs and government services and treated as second-class citizens in the country of 1.5 million.

The authorities deny the allegations and accuse Iran of fostering unrest that has seen demonstrators clash with security forces, who have been targeted by several bomb attacks. Tehran denies the allegations.

Bahrain emboldened

Human Rights Watch said in a statement that Manama is failing to create conditions necessary for a free election by "jailing or silencing people who challenge the ruling family" and banning all opposition parties.

A leader of dissolved opposition groups al-Wefaq said the rise of Saudi Crown Prince Mohammed bin Salman, also known as MBS, has emboldened Bahrain's authorities in their crackdown on dissent, which has included stripping scores of activists from their nationality.

"They couldn't go ahead with all the crackdown without the strong backing of the Saudi government. Mohammed bin Salman listens only to hardliners in Bahrain's ruling family," Ali Alaswad, who lives in self-exile in London and has been sentenced in absentia to life in prison, told Reuters news agency.

Government opponents say the space for political expression has been shrinking in the lead up to the election. Several activists, including a former legislator, were arrested last week for tweeting about boycotting elections, activists said.

"No one is barred from expressing their political views," said a government spokesperson.

"Bahrain is home to 16 political societies, the majority of which have put forward candidates for the upcoming elections, and the government fully supports open and inclusive political dialogue."

Some candidates have taken to social media to urge Bahrainis to vote as a patriotic duty.

"Those who don't participate will not be part of the national consensus or equation in Bahrain," said Ali Al Aradi, deputy president of Bahrain's House of Representatives.

Some opposition figures hope the outcry over the murder of prominent Saudi journalist Jamal Khashoggi in Riyadh's consulate in Istanbul last month could help strengthen more moderate voices in the region, including members of Bahrain's royal family who are open to dialogue with the opposition.

The killing of Khashoggi, a critic of Prince Mohammed, has drawn global condemnation and exposed Saudi Arabia's crackdown on dissent and aggressive foreign policy.

"Now if there's a real accusation from the US against Mohammed bin Salman, radical wings in Bahrain which don't want to work with the opposition will be weakened," Alaswad said.

But some analysts are sceptical.

"The killing of Khashoggi will simply serve to highlight that those wishing to highlight abuses face a much riskier task," said Marc Owen Jones of the Institute of Arab and Islamic Studies, University of Exeter.

"If anything it will have a chilling effect."