MALE (Reuters) - The Maldives police forced a halt to a presidential election on Saturday, in what supporters of the leading candidate said looked like a new coup as they started a sit-down protest that blocked parts of the capital.

Britain and the United States criticised the latest delay to elections in the Indian Ocean archipelago, which has been in turmoil since February 2012 when then President Mohamed Nasheed was ousted by mutinying police, military forces and armed demonstrators.

Just hours before polls were due to open for an election that Mr Nasheed looked set to win, pthe olice surrounded the secretariat of the Elections Commission.

"We cannot proceed with the election if police are obstructing it," Elections Commissioner Fuwad Thowfeek told a news conference, saying the police had "overstepped their authority".

Security forces cordonned off a central area of the capital Male that included the president's office and the supreme court, while Mr Nasheed's supporters blocked other streets with ropes, human chains, motorbikes and trucks, a Reuters reporter witnessed.

Thousands of Mr Nasheed's supoorters put down tarpaulins and settled in at two main road junctions as they started a sit-down protest that brought Male to a standstill. Mr Nasheed and Parliament Speaker Abdulla Shahid sipped tea with others in the crowd.

Mr Ahmed Khalid, 33, an artist at the protest, said: "The police are in control of this country. This is a coup."

Elections Commission member Ali Mohamed Manik said: "This is a dark day for democracy."

Mr Nasheed, who came to power in the Maldives' first free elections in 2008, looked set to return to office when he won the first round of an election on Sept 7, putting him in a good position to win a run-off vote set for Sept 28.

But that election was cancelled by the Supreme Court which cited fraud. International observers had said the election was free and fair. The court later ordered a fresh election by Oct 20 and a run-off by Nov 3, if required.

The Supreme Court upheld that decision in an early-morning ruling on Saturday after a request by the Election Commission.

There had been confusion over whether the election could go ahead, as some candidates had still not signed a new voter register.

The police said they would not support an election held "in contravention of the Supreme Court verdict and guidelines".

Chief Superintendent Abdulla Nawaz said he had acted due to concern about "any unrest that may occur in the country as a result of letting the election proceed".

Masked men fire-bombed a television station that backs Nasheed on Oct 7. Mr Nasheed's supporters have staged violent protests since he was ousted.

Elections Commissioner Thowfeek said he doubted the election could now be held before the end of the current presidential term on Nov 11.

President Mohamed Waheed called on the Elections Commission to hold discussions with all candidates to find a way to hold the election next week, on Saturday, Oct 26.

"The President urges all candidates and their political parties to find a solution to election disputes," a statement from Mr Waheed's office said.

A spokesman for Mr Nasheed's Maldivian Democratic Party, Mr Hamid Abdul Ghafoor, said there was "a clear mandate for the international community to intervene and to restrain these undemocratic forces... An interim arrangement has to be sought through international intervention".

A US diplomat in nearby Sri Lanka told reporters that the failure to hold the election "represents a real threat to democracy in the Maldives".

British Foreign Secretary William Hague said new delays "will be seen as nothing less than an attempt to frustrate the democratic process. This would undermine democracy, create greater uncertainty, further instability and damage the Maldives economy and international reputation".

Mr Nasheed's main election rival is Mr Abdulla Yameen, a half-brother of Mr Maumoon Abdul Gayoom, who ruled for 30 years and was considered a dictator by opponents and rights groups.

Holiday resort tycoon Gasim Ibrahim, who was finance minister under Mr Gayoom, also ran against Mr Nasheed.

Critical issues the new president will face include a rise in Islamist ideology, human rights abuses and a lack of investor confidence after current Mr Waheed's government cancelled the biggest foreign investment project, with India's GMR Infrastructure.

Mr Nasheed, who once held a Cabinet meeting underwater, with members in scuba gear, to highlight the danger of rising sea levels, won the Sept 7 polls with 45.45 per cent of the vote, short of the 50 per cent needed to avoid a run-off.