Former Maldives president and opposition leader Mohamed Nasheed has been sentenced to 13 years in prison on a terrorism charge after what rights groups said was a "flawed" trial, sparking international concern over the honeymoon islands.

Nasheed called on his supporters to take to the streets to protest against the sentence after a trial his party said was "blatantly politicized."

"I appeal to all of you today to stay courageous and strong, to confront the dictatorial power of this regime," his office quoted him as saying.

Nasheed, the Indian Ocean archipelago's first democratically elected leader, was convicted late on Friday under the country's tough anti-terror law of ordering the arrest of a chief judge when he was president in January 2012.

The 47-year-old was then taken to Dhoonidhoo prison on an island near the capital Male.

Nasheed's lawyers resigned before the end of his brief trial, saying it was biased and aimed at destroying his political career.

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It came amid growing opposition to the government of President Abdulla Yameen and will effectively prevent Nasheed from running for president at the 2018 elections.

The opposition has held regular night-time rallies over the past year to protest what they call growing authoritarianism, which has damaged the atoll nation's image as a tourist paradise.

Local media reported that police clashed with Nasheed's supporters across Male after the verdict.

Nasheed's Maldivian Democratic Party (MDP) said the verdict dealt a blow to the young democracy seven years after it embraced multi-party elections following three decades of rule by former strongman Maumoon Abdul Gayoom.

'Democracy jailed'

"Democracy jailed for 13 years on March 13, 2015," said MDP spokeswoman Shauna Aminath. "Nothing good will come out of this. It's a sad, sad, sad day for the Maldives," she added.

Regional power India, whose prime minister cancelled plans to visit the Maldives over the unrest, said it was "deeply concerned" and was monitoring the situation closely, while Washington, London and the European Union also expressed concern.

Rights group Amnesty International said the trial was "deeply flawed" and called the conviction "unsound."

"Rather than responding to international calls to strengthen the impartiality of the judiciary the government of the Maldives has proceeded with this sham trial for political reasons," said the group's Asia-Pacific Director Richard Bennett.

Yameen has denied that the move was politically motivated.

The MDP's Aminath said lawyers for the party would appeal Nasheed's conviction in the high court.

Nasheed resigned as Maldives leader in February 2012 after a mutiny by police and troops that followed weeks of protests over then-criminal court chief judge Abdullah Mohamed's arrest.

He was charged with corruption over the arrest three years ago but those charges were dropped last month. A few days later the prosecutor general had him re-charged and arrested under tougher anti-terror laws.

Nasheed, who was dragged to court on Feb. 22 and appeared to be in pain after being pushed and pulled by police, has maintained his innocence throughout.

His party said he was denied medical attention while in police custody.

International concern

The United States said in a statement after the verdict it was concerned at the "apparent lack of appropriate criminal procedures during the trial" against Nasheed.

State Department spokeswoman Jen Psaki said the US called on Male to "restore confidence in its commitment to democracy and the rule of law."

"We urge the government to ensure former president Nasheed's safety and well-being in custody, and we hope all Maldivians will express their views peacefully," she said.

Britain's junior foreign minister, Hugo Swire, tweeted that he was "deeply concerned".

"Many questions over due process and transparency," he wrote.

The EU said Nasheed's conviction raised "very serious questions about due process."

Nasheed came to power after building a pro-democracy movement with local and foreign support in opposition to the 30-year autocratic rule of Gayoom — Yameen's half-brother.

He formed the MDP in exile but then returned home to a hero's welcome, sweeping 54 percent of the vote in the 2008 elections.

He built a reputation as a campaigner against climate change, holding an underwater cabinet meeting in 2009 to press the world to cap the carbon emissions that cause global warming and threaten low-lying countries such as the Maldives.

But he was beaten to the presidency by Yameen in a run-off contest in 2013.

Current defense minister Moosa Ali Jaleel, who was head of the armed forces under Nasheed, is among four others facing terrorism charges.