The state of emergency gives security officials extra powers to arrest suspects, reports say.
The government has already suspended parliament and ordered the army to resist any moves by the Supreme Court to impeach President Abdulla Yameen.
A few hours after the emergency was declared, security forces entered the Supreme Court, a court spokesman said.
There were judges inside the court but the spokesman said he was unable to contact them.
The court had ordered the reinstatement of 12 MPs, which would see the opposition majority restored.
In a landmark decision, it also ruled the trial of former President Mohamed Nasheed unconstitutional.
Following the Supreme Court's decision on Friday, the government sacked the police commissioner who had pledged to enforce the court ruling.
It also ordered the detention of two opposition MPs who had returned to the Maldives, and warned that any court order to arrest the president for not complying with the Supreme Court ruling would be illegal.
Opposition MP Eva Abdulla said in a statement that the state of emergency was "a desperate move" that showed the government had "lost everything [including the] confidence of the people and institutions".
Mr Nasheed, who is currently in Sri Lanka, was the island's first democratically elected leader.
The Maldives has seen political unrest since he was convicted in 2015 under anti-terrorism laws for ordering the arrest of a judge. His conviction and 13-year sentence was internationally condemned, and he was given political asylum in the UK.
The Maldives previously declared a state of emergency in November 2015, after the government said it was investigating a plot to assassinate Mr Yameen.
That move also came two days before a planned protest by the country's main opposition, the Maldivian Democratic Party.
The Indian Ocean nation has been independent from Britain for 53 years. It was ruled for decades autocratically by then President Maumoon Abdul Gayhoom but became a multi-party democracy in 2008.
However 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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