* Turnout expected to be lower than 2012 polls
* Fighting in Benghazi overshadows voting
* Former PM Zeidan returns from Europe for vote
By Ahmed Elumami and Ayman al-Warfalli
TRIPOLI/BENGHAZI, Libya, June 25 (Reuters) - Fewer than half of the eligible Libyans voted in a parliamentary election on Wednesday overshadowed by violence, officials said, with the paltry turnout reflecting disillusion with chaos pervading since Muammar Gaddafi's overthrow in 2011.
Gunmen shot dead Benghazi lawyer Salwa Bugaighis, a prominent human rights activist who helped organize the first protests against Gaddafi when the uprising started in the eastern city. A security official said unknown people had entered her house to assassinate her.
At least four people were also killed in heavy clashes between Islamists and government forces in Benghazi, medics said, part of turmoil gripping the oil producer as the government is unable to control militias who helped oust Gaddafi and now defy state authority.
Turnout was much lower than in July 2012, the first free national vote in more than 40 years. Some 1.5 million were registered to vote, compared with 2.8 million in 2012, after rules were tightened.
Only 630,000 Libyans caste their vote, the election commission said. Live cameras from news channels in the main cities showed mostly empty polling stations.
The election was called last month as a way to strengthen central state authority after renegade army general Khalifa Haftar opened a campaign against Islamists in the east.
Some polling stations stayed shut for security reasons in the eastern Islamist hotspot of Derna, Kufra in the southeast where tribes regularly clash and the main southern city of Sabha, officials said.
As a result, up to 15 seats in the 200-strong assembly will remain vacant, Emad Al-Sayeh, head of the election commission, told reporters.
Tripoli's partners in the West had hoped the vote would help it to begin rebuilding a viable state.
Many Libyans fear the vote will produce just another interim assembly. A special body to draft a new national constitution has still not finished its work, leaving questions over what kind of political system Libya will eventually adopt.
Without a functioning government and parliament, Libya is struggling to impose authority over heavily armed former rebels, militias and tribes which carve out their own fiefdoms.
Libya also has a budget crisis. Protests at oilfields and shipping ports by armed militias have reduced oil production, the country's lifeline, to a trickle.
To discourage political infighting between parties, which paralysed decision-making and led to wrangling between two rival prime ministers in May, candidates must run as independents rather than as party representatives.
"I am participating again to vote for the House of Representatives so we can rebuild Libya," said Munira Ashour, a female teacher.
"I didn't vote for any congressional members who had nominated themselves again because they have had their chances without making any progress."
In Tripoli, former Prime Minister Ali Zeidan made a surprise appearance to cast his vote after returning from Europe, where he fled when parliament ousted him in March.
"We hope the elections will achieve their goals and that the House of Representatives will make a new start, better than the past," he told Reuters.
In Benghazi, polling stations opened despite clashes in one district when Islamist militants opened fire with heavy guns on a local security headquarters, security officials said. At least four people were killed and 30 wounded.
Divisions need to be bridged between Libya's West, once favoured by Gaddafi, and the neglected East where many demand autonomy and a greater share of the nation's oil wealth.
Electoral authorities tightened registration rules by requiring voters to show a national identification number, which many Libyans lack because of the collapse of state services.
Around 1,600 candidates were on the ballot, about 1,000 fewer than in the previous parliamentary vote. Some candidates put up street posters or platforms on social media. But the announcement of the election a month ago left little time before voting began, and there has been no real campaigning. (Additional reporting by Feras Bosalum and Ahmed Elumami; Writing by Ulf Laessing and Patrick Markey; Editing by Andrew Roche, Robin Pomeroy and Cynthia Osterman)