The Maldives Supreme Court has ordered the immediate release and retrial of the exiled former president Mohamed Nasheed and other opposition leaders.
In a statement, the court declared that their original trials violated constitutional and international law.
The court decision effectively restores the opposition majority in parliament, reinstating 12 members who had been stripped of their seats.
Opposition supporters took to the streets to celebrate the ruling.
Police in the capital, Male, used tear gas to disperse one group.
The country has seen political unrest since Nasheed, its first democratically elected leader, was convicted in 2015.
His conviction and 13-year sentence was internationally condemned, and he was given political asylum in the United Kingdom.
Former Vice-President Ahmed Adheeb Abdul Ghafoor and other opposition leaders were among the others named in the latest order.
It called for their immediate release and said the "questionable and politically motivated nature of the trials of the political leaders warrant a re-trial".
In a tweet, Nasheed called for President Abdulla Yameen to resign. The call was echoed in a joint statement issued by the country's opposition parties.
Nasheed told local television that he planned to return to Male, but not immediately.
"I will move forward wisely with the advice of the party and the united parties," he said.
The US ambassador to the Maldives urged the country's leader to respect the court's judgement.
A spokesman for the president's office said in a statement that the government questioned the ruling but would comply.
However, the chief of police has reportedly been fired following the ruling, Attorney General Mohamed Anil said.
The Indian Ocean nation has been independent from Britain for 53 years, during which time it was ruled for decades autocratically by then President Maumoon Abdul Gayhoom.
It became a multi-party democracy in 2008, but since President Yameen took power in 2013 it has faced questions over freedom of speech, the detention of opponents and the independence of the judiciary.
The nation is made up of 26 coral atolls and 1,192 individual islands, and is popular among foreigners as a luxury tourist destination.
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