* Police surround election offices to halt vote
* Britain, United States denounce "threat to democracy"
* Nasheed calls for Male shutdown by blocking streets
Maldives police forced a halt to a presidential election on Saturday, in what the leading candidate's supporters said was a new coup as he called on them to block the streets in protest.
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.
The election was due to be held on Saturday, after a vote in September was annulled over allegations of fraud.
However, there had been confusion over whether it could go ahead as some candidates had still not signed a new voter register in accordance with a Supreme Court ruling early on Saturday to allow the election.
Just hours before polls were due to open for the vote that Nasheed looked set to win, police surrounded the secretariat of the Elections Commission, forcing a delay condemned by the international community.
Police said they could not support an election held "in contravention of the Supreme Court verdict and guidelines".
Police 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".
Nasheed's supporters have staged violent protests since he was ousted, and masked men this month fire-bombed a television station that backs Nasheed, who came to international prominence in 2009 after holding a cabinet meeting underwater in scuba gear to highlight the threat of climate change.
"There has been a coup in the Maldives, and the coup backers, in order to maintain that coup, are committing bigger and bigger atrocities day after day," he told supporters staging a sit-in at two road junctions that brought Male to a halt.
"I call on you to block these streets ... Let us shut down Male. Male can't function, we must succeed."
Security forces cordonned off part of Male that included the president's office and the Supreme Court, while Nasheed's supporters blocked other streets with ropes, human chains, motorbikes and trucks, a Reuters reporter said.
Ahmed Khalid, 33, an artist at the protest, said: "The police are in control of this country. This is a coup."
"THREAT TO DEMOCRACY"
Elections Commissioner Fuwad Thowfeek told a news conference it could not proceed with the vote if police were obstructing it, saying officers had "overstepped their authority".
Thowfeek appeared on state television late on Saturday to say it would take a minimum of 21 days to amend the voter register again and the commission was in discussion with the government to potentially hold polls on Nov. 2 or Nov 9.
Elections Commission member Ali Mohamed Manik said: "This is a dark day for democracy."
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 current president's term expires on Nov. 11.
A spokesman for Nasheed's Maldivian Democratic Party, Hamid Abdul Ghafoor, called on Saturday for the intervention of world powers. "An interim arrangement has to be sought through international intervention," he said.
Commonwealth Secretary-General Kamalesh Sharma said: "I cannot stress firmly enough how critical it is for all state institutions and presidential candidates to cooperate in good faith to ensure that this election can take place as soon as possible."
A U.S. diplomat in nearby Sri Lanka told reporters 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".
Nasheed's main election rival is Abdulla Yameen, a half-brother of 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 Gayoom, was also running.
Critical issues the new president will face include a rise in Islamist ideology, human rights abuses and a lack of investor confidence after current President Mohamed Waheed's government cancelled the biggest foreign investment project, with India's GMR Infrastructure.