Police fired tear gas and baton-charged last night's protest on the main island of Male, said witnesses to what was the biggest show of support for Mohamed Nasheed since he was handed a 13-year term in March.
Nasheed's Maldivian Democratic Party (MDP) said one of its top lieutenants were among those arrested, while the head of the largest Islamist party in the mainly Sunni Muslim nation was detained.
But while authorities defended their use of force to break up the protest, Kerry added his voice to the growing chorus of criticism of President Abudulla Yameen's regime.
"We see even now how regrettably there are troubling signs that democracy is under threat in the Maldives where the former president Nasheed has been imprisoned without due process," said Kerry.
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"This is an injustice that needs to be addressed soon," he added on a visit to neighbouring Sri Lanka.
The MDP said 195 people had been arrested, accusing the security forces of responding to what it called a peaceful protest "with tear gas, baton charges (and) stun grenades".
She said the party's chairman Ali Waheed had been arrested along with Sheik Imran, leader of the main Islamic Adhaalath Party, who was an organiser of yesterday's protest.
The government responded in a statement by accusing the organisers of incitement, saying they had "called on all gathered to topple the government and confront the police".
Yameen came to power in late 2013 after controversially beating Nasheed in a run-off election despite trailing in the first round.
Nasheed, a climate change activist who was imprisoned during the three-decade rule of former strongman ruler Maumoon Abdul Gayoom, became the archipelago's first democratically elected leader in 2008.