Rights group Amnesty International says it is concerned at "repressive measures" being adopted by the government of the Maldives.
The opposition says its supporters have been detained and ill-treated
The group says there have been reports of mass arrests and ill-treatment of at least 110 opposition members recently.
Earlier this month, an opposition protest was called off after what the organisers called a "sustained campaign of government harassment".
The Maldivian government says it is disappointed by the Amnesty report.
The Amnesty report says, "Scores of detainees have remained held without charge, while at least 22 have been released after being charged with apparently unsubstantiated, politically motivated criminal offences."
'Brutal and illegal'
The report adds that "although the government denies being responsible for such human rights violations, testimonies and reports of arbitrary arrests and torture or other ill-treatment of the detainees abound".
Recently, activists from one of the main opposition parties, the Maldivian Democratic Party (MDP), called off a planned rally, saying that more than 100 of its activists had been arrested.
The party spokesman, Mohammed Latif, told the BBC "the arrests were carried out in a brutal manner, there was no legal basis for this drastic action".
President Gayoom is accused of introducing reform too slowly
The authorities accused the MDP of trying to stage the demonstration in order to overthrow the government.
In a statement issued in response to the Amnesty report, chief government spokesman Mohamed Hussain Shareef said: "We are hugely disappointed by Amnesty International's statement relating to alleged 'mass arrests and detentions' on and around 10 November.
"Amnesty International fails to recognise that the planned assembly was illegal and deemed to pose a real and present danger with regards to public safety and disorder."
It added: "Amnesty International has felt able, once again, to form opinion and pass judgment based on nothing more than hearsay and conjecture fed through to them from militant groups intent on causing maximum disruption to the president's constitutional reform programme."
Amnesty International says "the government's allegations against the opposition may have been made simply as a means to suppress the opposition's right to freedom of assembly and expression".
The report says this is not the first time the government of the Maldives has resorted to such allegations prior to a widespread crackdown on peaceful opposition activity.
"The government has so far failed to provide substantive evidence to support its allegations that the detainees had used, planned or advocated violence," it said.
Amnesty says, based on past experiences, it is concerned that the detainees are not likely to receive a fair trial and has urged the government to release them do not torture or ill-treat them.
The Maldives government has been under international pressure over its human rights record and says it is implementing democratic reforms.
Earlier this year, the government and opposition reached an agreement under which a number of opposition detainees have been freed.
But correspondents say relations between the two sides have recently taken a turn for the worse.