BAGHDAD, Dec 21 2005 (Reuters) - Iraq's Sunni Arab and secular parties threatened on Wednesday to boycott the new parliament after alleging massive fraud in last week's election, ramping up pressure on the triumphant Shi'ite Islamists to share power.

Sunni rebels, whose informal truce helped push turnout to 70 percent as insurgents pitched for a voice in the new, full-term legislature, warned they would intensify attacks if the Shi'ite Alliance held on to the lion's share of power.

The Electoral Commission, which opposition groups demanded be dissolved accusing it of bias, rejected calls for a rerun of the vote, saying complaints were numerous but unlikely to affect the overall result -- a view held by U.S. and U.N. officials.

With demands for a rerun or a substantial revision of the vote looking unlikely for now, lobbying by those disappointed with their shares of the vote seemed intended to back up calls for posts in a grand coalition government -- something the ruling Shi'ites have offered and Washington is encouraging.

Representatives of secular Shi'ite former prime minister Iyad Allawi and two major Sunni Arab groups, the Islamist-led Iraqi Accordance Front and the secular Iraqi Unified Front, along with other groupings, met on Wednesday to coordinate.

"We all agreed to contest and reject the results of the election," said Allawi aide Thaer al-Naqib. "We want the Electoral Commission dissolved and the election rerun."

"We will take to the streets if necessary," he told Reuters. "We might even not take up our seats in the new parliament and so any new government would be illegitimate."

Unified Front leader Saleh al-Mutlak said they would take their complaints not only to the Electoral Commission but also the Arab League, European Union and United Nations.

INSURGENT THREAT

Other Sunni Arab leaders have warned of a resumption of rebel violence if leaders whom they accuse of being puppets of Shi'ite, non-Arab Iran keep power; there were brief clashes with U.S. forces in the insurgent stronghold of Ramadi on Tuesday.

"The resistance will intensify and ... and much blood will be spilled if Iran's agents gain power," said Majeed al-Gaood, who says he speaks for rebel groups, from neighboring Jordan.

The United States, anxious to staunch the revolt as part of a strategy of withdrawing its 150,000 troops while leaving Iraq stable, has also made no secret of concerns over links between Shi'ite leaders and Washington's enemy Iran and over accusations pro-government sectarian militias are killing with impunity.

Though always denying having preferences for the election result, U.S. officials have made clear they want to see a broad, inclusive government across the sectarian and ethnic divides, even if the Alliance retains its slim majority in parliament.

The U.S. ambassador to Baghdad said pointedly on Tuesday that the Interior Ministry, target of much Sunni criticism over sectarian attacks, should not be run by a sectarian figure.

Provisional results offer an unclear picture of the new assembly, particularly since a complex process of redistributing votes is required to allocate 45 of the 275 seats. However, Shi'ite leaders believe they can expect at least 120 seats and possibly as many as the 140 or so they have at present.

Electoral Commission chief Hussein al-Hindawi told a news conference that 10.9 million voters took part on Thursday, putting national turnout at 70 percent, much higher than the 58 percent who participated in January's ballot, when many in the Sunni Arab minority stayed away from the polls.

Among the regional votes, turnout in western Anbar province, where insurgents are strong in towns like Ramadi, hit 55 percent. That compared to just two percent in the January 30 election for an interim assembly.

(Additional reporting by Suleiman al-Khalidi in Amman and Alastair Macdonald in Baghdad)

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