Human Rights Watch has called on Jordan to "respect the right to free expression" in the run-up to next month's general election.
The New York-based rights group says recent arrests of opposition activists shows the authorities are trying to prevent free speech and public protest.
Jordan's King Abdullah II has called for free and transparent polls.
But a BBC correspondent says it is unclear how fair they will be without the participation of opposition groups.
"Recent incidents of censorship and arrests of critics cast doubt on the open contest of ideas necessary for the 'transparent and fair' elections King Abdullah promised," said HRW spokesman Christoph Wilcke.
"Jordanian authorities are trying to delegitimise the opposition, but instead they are delegitimising the elections," he added in a statement released by the group.
His report pointed to the detention of 18 members of the opposition Popular Unity party, who were demonstrating outside the prime minister's office to demand electoral reform. The authorities said they had not obtained a permit for the gathering.
In another recent incident, more than 30 youths were arrested for backing a boycott of the 9 November election that has been called by the Islamic Action Front, the country's largest opposition party.
The IAF leadership ordered the boycott in protest against the constituency boundaries which it says over-represent pro-government rural areas at the expense of urban areas regarded as Islamist strongholds.
The current electoral system favours Bedouins in sparsely populated rural districts who are the backbone of King Abdullah's support at the expense of urban areas where there are high numbers of Jordanians of Palestinian origin, says the BBC's Dale Gavlak in Amman.
Jordan has been the main destination for Palestinian refugees fleeing the conflict with Israel, which means the country maintains a delicate demographic balance between people of Palestinian origin and other Jordanians, our correspondent says.
Without the participation of a number of opposition candidates, it is not clear how inclusive, free and fair the election will really be, she adds.