PORT-AU-PRINCE, Haiti (Reuters) - Haitians lined up to vote on Sunday for the first time in four years, in a test of stability for an impoverished country continually rocked by political turmoil.

Men armed with rocks and bottles attacked polling stations in the capital of Port-au-Prince and at least three were forced to close, according to election officials.

The Caribbean nation of about 10 million people has struggled to build a stable democracy ever since the overthrow of the dictatorship of the Duvalier family, who led Haiti from 1957 to 1986, and ensuing military coups and election fraud.

The country was also devastated by an earthquake in 2010 that flattened large parts of the capital, including the presidential palace, killings tens of thousands of people.

Haiti's parliament dissolved in January after scheduled legislative elections in 2011 and 2014 were canceled. The previous presidential election, which began in 2010 and carried over into 2011, was marred by low turnout, violence and a delayed, disputed runoff.

Since January, the 119-member Chamber of Deputies has sat empty, and the Senate, with only 10 of its 30 members, has failed to field a quorum. Sunday's election will select a new lower house of parliament and two-thirds of the Senate.

"Credible, inclusive, translucent and fair elections are key to long-term stability in Haiti," Sandra Honore, the special representative of the U.N. secretary general in Haiti, told Reuters on the eve of the vote.

President Michel Martelly, who cannot run for re-election, has dozens of candidates running throughout the country under the so-called Haitian Bald Headed Party (PHTK), named after his famously smooth scalp. The Verite Party of former president Rene Preval and the Lavalas Family party, linked to twice-deposed former president Jean-Bertrand Aristide, are also running candidates.

Results are expected in six to 10 days and runoffs are set for Oct. 25, the same day as the first round of presidential voting.

Following a violent campaign, the election will test the Haitian National Police, which has taken full control of security during election season from a U.N. peacekeeping force.

The National Network for the Protection of Human Rights reported five election-related assassinations in the last month and 26 wounded.

At one precinct at a school in the capital near the gang-infested Belair neighborhood, voting was suspended after it came under attack, said Guerline Benjamin, an election supervisor at Ecole Nationale Isidore Boisrond.

"They came in with force and threw rocks and bottles. They ripped up the (voting) materials," Benjamin said. The mob overwhelmed six Haitian policemen guarding the school, she said.

It was unclear who was trying to disrupt the voting, though people in the crowd blamed Martelly supporters.

Elsewhere some polling stations opened late but voting got underway smoothly, with voters expressing skepticism that the results would bring much change.

"We know a real change won't come from the sky," said Joel Joseph, 27, after he exited his polling station.

Election observers expect low turnout, in part because of widespread confusion about where people might be registered to vote.