COLOMBO (AFP) - The Maldives faces a final chance to avoid a constitutional crisis and secure its five-year democracy this weekend when it makes a third attempt to hold a long-delayed presidential election.

The United States and Britain have warned that failure to go ahead with the re-scheduled ballot on Saturday will damage the Indian Ocean atoll nation and its fragile tourism-dependent economy.

Western and Indian diplomats have come to view the annulment of a first round of elections that took place on September 7 and police action to prevent a second vote on October 19 as deliberate moves to block opposition leader Mohamed Nasheed.

"Remnants of the former regime will be quite keen to keep Nasheed out of power," a Colombo-based diplomat told AFP on condition of anonymity. "There are some who fear retribution in the event of a change of government." The Maldives' 2008 constitution, which ended 30 years of one-party rule by former autocrat Maumoon Abdul Gayoom, stipulates that a new president must be elected by November 11.

It makes no provisions for interim arrangements, meaning the nation of 350,000 Sunni Muslims will face a power vacuum if no one is chosen to replace President Mohamed Waheed, who came to power in controversial circumstances last year.

"As the current government's mandate expires on November 11, time is of the essence," the US embassy said in a strongly worded statement that also condemned calls for foreign intervention.

Nasheed, the country's first elected president, resigned in February 2012 following demonstrations and a mutiny by security forces that he denounced as a coup engineered by Waheed and former strongman Gayoom.

He swept to victory in the first round of elections on September 7 with 45 percent of the vote and was the front-runner in a second round run-off against Gayoom's half-brother Abdulla Yameen scheduled for three weeks later.

But the run-off never happened because the Supreme Court annulled the first round after a defeated candidate linked to Gayoom complained of fraud.

International and domestic election observers had adjudged the voting as free and fair.

The top court, made up of judges appointed during Gayoom's 30-year rule, also decreed that all competing parties had to sign off on the electoral rolls, paving the way for further delays and challenges.

The judicial interventions and perceived complicity of President Waheed, an unelected former UN diplomat who won just 5.0 percent of the vote in September, have led to rising criticism from Western powers and regional heavyweight India.

"These legal challenges increasingly appear to be aimed at preventing the people of the Maldives from having their say at the ballot box," junior British foreign minister Hugo Swire said last week.

Opposition activists in the Maldives have publicly called for foreign intervention if elections are blocked for a third time on Saturday, but the United States warned all parties against inflaming tensions.

"Extra-legal manoeuvring and calls for military intervention are neither appropriate, nor acceptable under international law," the US statement said.

Western-educated Nasheed is hoping that recent turmoil, which he has depicted as a fight between pro-democracy forces and the old regime, will help him win an outright victory of more than 50 percent on Saturday.

If not, a run-off is scheduled for November 16, which would still leave the country without a leader for five days.

Gayoom's half brother Yameen is set to stand again on Saturday, as is one of the country's wealthiest businessmen, Qasim Ibrahim, who is a tourist resort tycoon.

Ibrahim initiated the court action seeking the cancellation of the September results.

Waheed, who served as vice-president under Nasheed before replacing him in February 2012, has denounced foreign pressure on the country.

He held a meeting with all three candidates on Wednesday, with a statement saying it was "a very crucial period" for the country, which would face "many challenges" if a new president were not elected by Monday.

After coming last in September's vote, he has withdrawn from Saturday's ballot.