MALE, Maldives (AFP) - Opposition leader Mohamed Nasheed topped Saturday's bitterly-contested presidential elections, but failed to secure an outright majority to avoid a run-off, official results showed.

Mr Nasheed received 45.08 per cent of the popular vote and is set for a run-off with Abdulla Yameen, the half brother of former autocrat, Maumoon Abdul Gayoom, who garnered 31.21 per cent of the vote.

The official count showed that a further 3.4 per cent of the vote was yet to be declared, but even if all of it went to Mr Nasheed he was still short of the 50 per cent needed to win outright to avoid a run-off scheduled for Sunday.

The election was observed by over 2,000 local and foreign monitors across the archipelago of 1,192 tiny coral islands.

However, a run-off planned for Sunday in the event no candidate wins more than 50 percent of the vote may be in doubt, Chief Elections Commissioner chief Fuwad Thowfeek told reporters in the capital Male.

The two other candidates, Abdulla Yameen and Qasim Ibrahim, were yet to approve voter lists, a requirement by the Supreme Court for a run-off to take place, he said, threatening a constitutional crisis.

Under the constitution, a new president must be sworn in by Monday in the Sunni Muslim nation of 350,000.

The Maldives, whose turquoise seas and white beaches have long been a tourist draw, has been the focus of intense US-led diplomatic pressure since judges annulled results of a September 7 vote.

When new polls were scuppered six weeks later, suspicions grew that authorities were determined to prevent Mr Nasheed from returning to power at any price.

The 46-year-old - a one-time political prisoner and environmental activist - won the first multi-party elections in 2008, ending 30 years of iron-fisted rule by Maumoon Abdul Gayoom.

But after clashing with key institutions, including the judiciary and security forces, he was forced to resign in February 2012.