N'DJAMENA, May 5 (Reuters) - African Union and other observers gave a seal of approval on Friday to presidential polls in Chad, but assertions the vote was free and fair and turnout high astonished Western diplomats and journalists.

Although official results are not due for a week, President Idriss Deby's re-election for a third five-year term is seen by most observers as a foregone conclusion following Wednesday's one-sided poll in the landlocked, central African oil producer.

While his supporters gave varying voter turnout estimates ranging from 30 percent to more than 60 percent, some diplomats put it closer to only 10 percent. The African observers estimated turnout at more than 60 percent in some areas.

The election, which went ahead peacefully despite a threat of disruption by rebels fighting to topple Deby, was boycotted by leading opposition parties. The president had faced four weaker challengers, two of whom were government ministers.

Opposition and rebel figures condemned the polls as a fraudulent sham aimed at extending Deby's nearly 16-year rule.

"Notwithstanding some deficiencies of a minor organizational nature, the international observers consider that the first round of the May 3, 2006 presidential elections was free, transparent and fair," representatives of the African Union and 19 African non-governmental organizations said in a statement.

"In most of the polling stations visited, a strong participation by voters was noted," said the statement, read at a news conference in the capital N'Djamena.

The statement drew gasps of incredulity from journalists and diplomats present, who had seen only a thin dribble of voters throughout Wednesday in the dusty capital, as many Chadians appeared to heed opposition calls for a boycott.

"It's a whitewash," said one Western diplomat, who asked not to be named. "I saw no massive turnout," said another.

But despite pressure from dumbfounded journalists, who wondered whether they had witnessed the same election, the African observers stood by their assessment.

"We in the African Union saw a massive turnout in the elections," Silikam Isabelle, a national assembly deputy from Cameroon and designated AU observer, said.

"I don't think so," commented a Western diplomat.

The 81 African observers were among a limited number of formal external monitors who accepted a government invitation to the election. There were no official observer missions from the European Union or the United States, although European and U.S. diplomats based in N'Djamena visited polling stations.

DOOR OPEN FOR DIALOGUE?

The low, unenthusiastic turnout witnessed by journalists and diplomats appeared to offer only a flimsy popular endorsement of Deby, who seized power in a 1990 revolt and won elections in 1996 and 2001.

Few believed the latest polls offered peace to the former French colony, which has had a history of clan-based ethnic feuding and civil war since independence in 1960.

Deby, who accuses neighbour Sudan of backing the rebels, has offered opponents a post-election dialogue, including a possible amnesty, if they accept the election result.

"The door is open for dialogue," said Mahamat Hissene, the campaign director of Deby's ruling Patriotic Salvation Movement (MPS), which has said it is sure of the president's re-election.

But, rejecting opposition calls for Deby to concede power, he added "Sharing is not democracy".

Rebel spokesmen, who portray Deby as a dictatorial and corrupt ruler, say they have no faith in his offer of dialogue and most analysts believe armed resistance will continue.