The London-based human rights group said police in the Maldives carried out the mass arrests on Friday, ahead of a peaceful political rally planned for Saturday in the capital island of Male.
"President Maumun Abd al-Gayum's promises of reforms to protect human rights are in sharp contrast to the arrests made by the police," Amnesty said.
"Our message to the government is clear: release these detainees now; ensure their well-being while in custody; reform the laws that allow the detention of prisoners of conscience; and ensure that no such arrests take place in the future."
Amnesty said the number of those detained was not immediately clear, but reports from the Maldives, south Asia's most expensive tourist destination, said up to 20 people had been rounded up during the pre-dawn swoop on Friday.
Women and children
The Maldivian Democratic Party (MDP) said women and children were among those arrested on Friday.
Gayum, who was re-elected president for his sixth five-year term in October, has been criticised by international rights organisations for suppressing dissent, a charge the administration has strongly denied.
President Gayum has been
criticised for suppressing dissent
Gayum two months ago set up a human rights commission in a bid to deflect criticism over abuses in the Maldives which is home to about 270,000 Muslims.
The Paris-based media watchdog, Reporters Without Borders (RSF) last week demanded the early release of three "cyber dissidents" in the Maldives, who ran an on-line newsletter and were jailed on charges of attempting to overthrow Gayum.
Prisoners of conscience
Last month, Amnesty raised the plight of five "prisoners of conscience" whom it said continued to be held "in gross violation of their fundamental rights to freedom of expression despite developments in recent months promising to improve the human rights situation in the country".
The prisoners include Muhammad Zaki, Ahmad Ibrahim Didi and Fatima Nisreen, arrested two years ago and sentenced in evidently unfair trials to long periods of imprisonment, said Amnesty in a statement.
"We urge President Abd al-Gayum to release these prisoners of conscience immediately and unconditionally... no move towards reform can gain credibility while these prisoners of conscience remain in detention"
The three have been detained since the end of January 2002 on charges related to their involvement in the production of a Internet e-mail magazine, Sandhaanu, publishing articles critical of the government and circulated widely amongst Maldivians.
They were sentenced to long terms of imprisonment in July 2002 following "grossly unfair trials", says Amnesty.
Recent reports suggest that Zaki and Didi's sentences (life imprisonment) may have been reduced 15 years'.
Fatima Nisreen's sentence of 10 years has reportedly been halved and she has been banished to a remote island where she is to spend the rest of her sentence - still a form of imprisonment.
Two other prisoners of conscience, Naushad Wahid, sentenced to 15 years, and Ibrahim Farid, reportedly held without charge or trial, have been in detention since December 2001 and May/June 2002 respectively.
"We urge President Abd al-Gayum to release these prisoners of conscience immediately and unconditionally," Amnesty emphasised. "No move towards reform can gain credibility while these prisoners of conscience remain in detention."