No details were given about the investigation or any charges.
The turmoil began when President Abdulla Yameen refused to obey a court order to release political dissidents.
The opposition has called the government moves a "purge" and there has been international condemnation.
Maldives is a nation made up of 26 coral atolls and 1,192 individual islands, and tourism is a vital part of its economy.
What's happening in the Maldives?Last week, the Supreme Court ordered the release of a group of opposition politicians. It also ruled that the 2015 trial of former President Mohamed Nasheed, who is living in exile, had been unconstitutional.
The country's police commissioner said he would enforce the court's ruling - in response, the government of President Adulla Yameen sacked him.
The army has now been ordered to resist any attempt to impeach or remove President Yameen.
The crisis escalated on Monday when a state of emergency was declared, giving a range of powers to the security forces to make arrests, and banning public gatherings.
Former President Maumoon Abdul Gayoom, who has allied himself with the opposition, was detained at his home.
In a video posted online the ex-president addresses his supporters (in Divehi), saying that he has not "done anything to warrant arrest" and urging them to "remain strong".
Police were also dispatched to the Supreme Court.
Tension turns to fear
Last week's Supreme Court ruling very much caught the government on the back foot but it had been difficult to see how they would manage to avoid implementing it in the face of domestic and international pressure. With drastic measures, it seems.
President Yameen clearly saw a threat to his rule and is pulling no punches in fending off the challenge.
The mood has turned from tense to fearful. More arrests are expected to take place overnight.
Maldivians are passing around messages telling one another to "be safe". With many constitutional rights suspended, there is little protection.
What has the reaction been?
Mr Nasheed - the country's first democratically elected leader whose trial was at the centre of the Supreme Court's ruling - told BBC News that the government's actions were "brazenly illegal" and amounted to a coup.
"Maldivians have had enough of this criminal and illegal regime," he said. "President Yameen should resign immediately."
Since President Yameen took power in 2013 the country has faced questions over freedom of speech, the detention of opponents and the independence of the judiciary.
The US State Department said it was "troubled and disappointed" by the developments.
America stands with the people of Maldives. The Maldivian government and military must respect the rule of law, freedom of expression, and democratic institutions. The world is watching.
It accused police of failing to obey a lawful court ruling, and said President Yameen had "jailed or exiled every major opposition political figure".
The US National Security Council, meanwhile, warned in a tweet that "the world is watching".
Boris Johnson, foreign secretary of the UK, the former colonial power, called on President Yameen to lift the state of emergency.
"The damage being done to democratic institutions in Maldives and the sustained misuse of process in Parliament is deeply worrying," he said in a statement.
What does this mean for tourists?
The political crisis comes during the country's peak tourism season when tens of thousands of foreigners visit the Maldives' tropical beaches.
China, the United States, India and the UK are among those issuing warnings to travellers in the Maldives,
The UK's Foreign and Commonwealth Office said British nationals in the capital, Malé, should "exercise caution and avoid any protests or rallies", but that outlying islands and resorts - and the international airport - were not affected.
Tourism was worth $2.7bn (£1.9bn) to the Maldivian economy in 2016. It is the biggest industry in the country - home to just 400,000 people - and could be hit hard if the crisis worsens and puts people off travelling.
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