VIENNA (Reuters) - Electoral officials broke one of Austria's vote-counting rules in last month's presidential poll because it was almost impossible to implement, witnesses told the Constitutional Court on Monday as it examined a challenge to the result by the far-right Freedom Party (FPO).

FPO candidate Norbert Hofer, aspiring to be the first far-right head of state to be elected in a European Union country, lost narrowly to a former leader of the Greens Party. In announcing its challenge, the party cited irregularities on a "more than frightening" scale.

A lawyer for winning candidate Alexander Van der Bellen described the court testimony so far as not giving rise to any suspicions of manipulation.

"No manipulation is in any way apparent and it (the count) was largely carried out correctly," lawyer Maria Windhager told reporters outside the courtroom.

No other officials, when questioned, said they believed there had been any manipulation of the vote.

The FPO challenge focuses on most of the more than 700,000 postal ballots cast, which swung the election in favor of independent Van der Bellen. His margin of victory was roughly 31,000 votes, or less than one percentage point.

The Constitutional Court must decide whether the law was broken in such a way that the result might have been affected. If so, it can order remedies including a re-run. It aims to rule before Van der Bellen's inauguration scheduled for July 8.

The most widespread irregularity according to the FPO was that postal ballots were processed or counted before 9 a.m. the day after the Sunday election. The Interior Ministry agrees officials in several districts failed to wait until then.

"Because ... of the enormous amount of postal voting ballots it would barely have been possible to provide a result in time starting on the Monday at 9 a.m.," Innsbruck-Land voting district head Wolfgang Nairz told the court.

At the start of four days of public proceedings in which the court plans to hear around 90 witnesses, Nairz said his voting district had authorised him years ago to process ballots early. The candidates' lawyers dispute whether that is allowed.

An official from another of Austria's 117 voting districts, Southeastern Styria, told the court postal ballots were processed early there, too, because there would not have been enough people to handle them on the day after the election.

Other irregularities alleged by the FPO include that unauthorized people counted ballots. The Interior Ministry has countered that and other arguments by saying FPO observers signed documents stating the count was carried out correctly.

Election observers from various parties said they signed those documents without reading them.

"I assumed everything was in order," an FPO observer in Nairz's district, Johann Grassmair, said, adding that he did not know of the 9 a.m. rule at the time. "I am aware now."

(Reporting by Francois Murphy and Kirsti Knolle; editing by Ralph Boulton)