In a court hearing late last night, Nasheed, who was arrested on February 22 over the detention of a judge in 2012, was charged under the Anti-Terrorism Act 1990.
The former president, the first democratically-elected leader of the Indian Ocean archipelago, has a constitutional right of appeal to the High Court of the Maldives.
"The United States is concerned with the apparent lack of appropriate criminal procedures during the trial in Maldives against former President Mohamed Nasheed, who was found guilty today and sentenced to 13 years in prison.
"This includes the denial of legal representation to former President Nasheed during the first hearing and concerns regarding the lack of impartiality and independence of the judges," the US said.
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An EU statement said "The conviction of former President Mohamed Nasheed for charges under anti-terrorist legislation raises very serious questions about due process of law and risks undermining people's trust in the independence of the judiciary.
"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," Amnesty International said.
47-year-old Nasheed resigned as the Maldives' leader in February 2012 after a mutiny by police and troops that followed weeks of protests over the arrest of judge Abdullah Mohamed on corruption allegations.