The focus will be on turnout and the performance of the Islamic Action Front (IAF), the political arm of Jordan's Muslim Brotherhood.
Experts expect the party to clinch around 20 seats in the 130-seat parliament, which would make it the largest opposition force.
The vote comes as Jordan wrestles with the spillover of wars in neighbouring Syria and Iraq and the burden of hosting hundreds of thousands of refugees.
The kingdom is a member of the US-led coalition battling jihadists in both neighbouring countries and was the target of a June 21 suicide bombing claimed by the Islamic State group that killed seven border guards.
In Jordan, King Abdullah II can appoint and sack military and intelligence chiefs, senior judges and members of parliament's upper house without government approval.
By midday nearly 600,000 of the 4.1 million registered voters had cast a ballot, the electoral commission said. In the last election in 2013, just over 1.2 million voted.
The Islamist-led opposition welcomed the early turnout but complained of "several" irregularities, including vote buying, which it said had taken place openly outside polling stations.
The interior ministry said 50,000 policemen had been mobilised to ensure security at polling stations.
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