(Adds province-wide curfew in 11th paragraph)
RAMADI, Iraq, Feb 2 (Reuters) - Tribal sheikhs who helped drive al Qaeda militants out of Western Iraq threatened on Monday to take up arms against the provincial government because of what they said was fraud in Saturday's provincial polls.
The election was the most peaceful in Iraq since the fall of Saddam Hussein, but there has been tension in the west of the country between Sunni Arab groups, many of whom boycotted the last provincial ballot in 2005.
Anbar province, Iraq's vast western third, was once the heartland of the Sunni Arab insurgency against U.S. troops but is now largely quiet, thanks to tribal guard units known as Awakening councils that helped drive out al Qaeda militants.
In one of the toughest-fought contests of the election, the tribes have challenged the Iraqi Islamic Party (IIP), a Sunni religious party which has run the province since 2005.
With the IIP claiming the results will keep it in power, Awakening leaders alleged fraud in the voting.
"We threatened the electoral commission not to allow fraud. We said we will transform from a political entity to an armed wing against the electoral commission and the IIP because we discovered fraud," Awakening movement head Sheikh Ahmed Abu Risha told Reuters.
Hamid al-Hais, head of the Anbar Tribes list in the election, travelled to Baghdad to lodge a protest.
"We will set the streets of Ramadi ablaze if the Islamic Party is declared the winners of the election," he told Reuters, referring to Anbar's provincial capital. "We will make Anbar a grave for the Islamic Party and its agents. We will start a tribal war against them and those who cooperate with them."
Electoral authorities say official preliminary results will not be announced for days and final results may take several weeks. The IIP says it came in first and expects to hold onto power in the province.
"We are convinced that we will be the first in Anbar and we will make a coalition with any entity that wants to work with us," said Khalid Mohammed al-Alwani, IIP head in Falluja, the Anbar town where there were fierce battles between U.S. forces and insurgents in 2004.
Authorities imposed an overnight curfew on Monday to prevent outbreaks of violence. Colonel Mahmoud al-Esawi, chief of police in Falluja, said the curfew would be in force from 10 p.m. to 5 a.m. (1900-0200 GMT) throughout the province.
Witnesses said supporters of both sides fired guns into the air late on Sunday to celebrate perceived victories -- IIP supporters near the governor's office in Ramadi and Awakening movement supporters just outside the town.
No one was hurt, but the gunfire rattled throughout the city for about two hours.
The electoral commission has said the election took place without major violations. However, it has acknowledged that thousands of people were unable to vote because they could not find their names on registration lists.
The Kurdish regional government said it would file a formal complaint because thousands of Kurds were unable to vote. (Additional reporting by Wisam Mohammed in Baghdad; Writing by Peter Graff; Editing by Andrew Dobbie)
Our Standards: The Thomson Reuters Trust Principles.