Israeli police grilled Prime Minister Ehud Olmert for three hours on Friday over graft allegations that have dogged his term in office, two days after his surprise resignation announcement.

It was the fourth time since May that anti-fraud squad officers have interviewed Olmert, 62, at his official residence in Jerusalem in connection with the allegations of wrongdoing, which he has strongly denied.

Police remained tight-lipped but the questioning had been expected to focus on claims Olmert illegally received electoral and other funding from a wealthy US financier and that he sent out multiple bills for the same overseas trips.

The investigators asked the prime minister to explain allegedly incriminating documents, some writen by his own hand, Israeli media reported.